Could you please provide links to the failing builds when this happens?

I'm not seeing these failures on:
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Mac%20FYI%20Retina%20Release%20%28AMD%29?limit=200

The two flakes I do see are:
WebglConformance_conformance_rendering_multisample_corruption
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Mac%20FYI%20Retina%20Release%20%28AMD%29/11477
https://chromium-swarm.appspot.com/task?id=426347b3161d1310&refresh=10&show_raw=1

and

WebglConformance_conformance2_textures_misc_tex_mipmap_levels
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Mac%20FYI%20Retina%20Release%20%28AMD%29/11485
https://chromium-swarm.appspot.com/task?id=42647e3abeb98f10&refresh=10&show_raw=1

In both cases it looks like (though the multisample_corruption bug has poor logging) the canvas rendered black instead of the intended color.

These two flakes happened on different bots:
https://chromium-swarm.appspot.com/bot?id=build403-m4&sort_stats=total%3Adesc
https://chromium-swarm.appspot.com/bot?id=build489-m4&sort_stats=total%3Adesc

and there's no indication that the bots are failing lots of tests.

Not clear how to proceed with this; perhaps monitor and close if things appear stable.

It's on the NV bot, not the AMD one, e.g.,
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Mac%20FYI%20Retina%20Release%20%28NVIDIA%29/4727

Here's the flakiness dashboard for this test:

https://test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=gpu_tests.webgl_conformance_integration_test.WebGLConformanceIntegrationTest.WebglConformance_conformance2_textures_canvas_tex_3d_rg16f_rg_float&testType=webgl2_conformance_tests

Oops, thanks, I misread the description.

From that failing run:
https://chromium-swarm.appspot.com/task?id=4270be613a0eb610&refresh=10&show_raw=1

the renderer hung, waiting for the GPU process, during a ReadPixels inside Skia. The cause is likely that the GPU process either hung or crashed. We should improve our test harness to be able to force a GPU process crash when this happens so that we can see stack traces there, too.

Here's an excerpt of the stack from the renderer's hung thread:

  	Thread 0
  	 0  libsystem_kernel.dylib + 0x1320a
  	    rax = 0x000000000100001f   rdx = 0x0000000000000000
  	    rcx = 0x00007ffee89bf8e8   rbx = 0x0000000000000006
  	    rsi = 0x0000000000000006   rdi = 0x00007ffee89bfa90
  	    rbp = 0x00007ffee89bf940   rsp = 0x00007ffee89bf8e8
  	     r8 = 0x0000000000014f7b    r9 = 0x0000000000000000
  	    r10 = 0x0000000000000000   r11 = 0x0000000000000206
  	    r12 = 0x0000000000000000   r13 = 0x0000000000000006
  	    r14 = 0x00007ffee89bfa90   r15 = 0x0000000000000000
  	    rip = 0x00007fff76d3220a
  	    Found by: given as instruction pointer in context
  	 1  Chromium Framework!__ZN4base13WaitableEvent14TimedWaitUntilERKNS_9TimeTicksE + 0x1d2
  	    rbp = 0x00007ffee89bfb30   rsp = 0x00007ffee89bf950
  	    rip = 0x000000010cd80222
  	    Found by: previous frame's frame pointer
  	 2  Chromium Framework!__ZN4base13WaitableEvent4WaitEv + 0x29
  	    rbp = 0x00007ffee89bfc70   rsp = 0x00007ffee89bfb40
  	    rip = 0x000000010cd7ffe9
  	    Found by: previous frame's frame pointer
  	 3  Chromium Framework!__ZN3gpu14GpuChannelHost4SendEPN3IPC7MessageE + 0x3ba
  	    rbp = 0x00007ffee89bfe30   rsp = 0x00007ffee89bfc80
  	    rip = 0x00000001094c6c0a
  	    Found by: previous frame's frame pointer
  	 4  Chromium Framework!__ZN3gpu22CommandBufferProxyImpl4SendEPN3IPC7MessageE + 0x1af
  	    rbp = 0x00007ffee89bffa0   rsp = 0x00007ffee89bfe40
  	    rip = 0x00000001094c20ff
  	    Found by: previous frame's frame pointer
  	 5  Chromium Framework!__ZN3gpu22CommandBufferProxyImpl23WaitForGetOffsetInRangeEjii + 0x1a1
  	    rbp = 0x00007ffee89c0130   rsp = 0x00007ffee89bffb0
  	    rip = 0x00000001094c2451
  	    Found by: previous frame's frame pointer
  	 6  Chromium Framework!__ZN3gpu19CommandBufferHelper23WaitForGetOffsetInRangeEii + 0xca
  	    rbp = 0x00007ffee89c0290   rsp = 0x00007ffee89c0140
  	    rip = 0x000000010920502a
  	    Found by: previous frame's frame pointer
  	 7  Chromium Framework!__ZN3gpu19CommandBufferHelper6FinishEv + 0x5d
  	    rbp = 0x00007ffee89c03f0   rsp = 0x00007ffee89c02a0
  	    rip = 0x000000010920513d
  	    Found by: previous frame's frame pointer
  	 8  Chromium Framework!__ZN3gpu18ImplementationBase10WaitForCmdEv + 0x2b
  	    rbp = 0x00007ffee89c0420   rsp = 0x00007ffee89c0400
  	    rip = 0x000000010e75ee3b
  	    Found by: previous frame's frame pointer
  	 9  Chromium Framework!__ZN3gpu5gles219GLES2Implementation10ReadPixelsEiiiijjPv + 0x861
  	    rbp = 0x00007ffee89c0650   rsp = 0x00007ffee89c0430
  	    rip = 0x000000010e702831
  	    Found by: previous frame's frame pointer
  	10  Chromium Framework!__ZZN12GrGLFunctionIFviiiijjPvEEC1IZN12_GLOBAL__N_19gles_bindIvJiiiijjS0_EEES_IFT_DpT0_EEMN3gpu5gles214GLES2InterfaceEFS6_S8_EPSD_PNSB_14ContextSupportEEUliiiijjS0_E_EES6_ENUlPKviiiijjS0_E_8__invokeESL_iiiijjS0_ + 0x6f
  	    rbp = 0x00007ffee89c06b0   rsp = 0x00007ffee89c0660
  	    rip = 0x00000001094d265f
  	    Found by: previous frame's frame pointer
  	11  Chromium Framework!__ZN7GrGLGpu12onReadPixelsEP9GrSurfaceiiii11GrColorTypePvm + 0x5ee
  	    rbp = 0x00007ffee89c07f0   rsp = 0x00007ffee89c06c0
  	    rip = 0x000000010d3d22ae
  	    Found by: previous frame's frame pointer
  	12  Chromium Framework!__ZN5GrGpu10readPixelsEP9GrSurfaceiiii11GrColorTypePvm + 0x161
  	    rbp = 0x00007ffee89c0850   rsp = 0x00007ffee89c0800
  	    rip = 0x000000010d2c6a81
  	    Found by: previous frame's frame pointer
  	13  Chromium Framework!__ZN13GrContextPriv17readSurfacePixelsEP16GrSurfaceContextiiii11GrColorTypeP12SkColorSpacePvmj + 0x1149
  	    rbp = 0x00007ffee89c0ab0   rsp = 0x00007ffee89c0860
  	    rip = 0x000000010d2b0f19
  	    Found by: previous frame's frame pointer
  	14  Chromium Framework!__ZN16GrSurfaceContext10readPixelsERK11SkImageInfoPvmiij + 0x10d
  	    rbp = 0x00007ffee89c0b30   rsp = 0x00007ffee89c0ac0
  	    rip = 0x000000010d31f32d
  	    Found by: previous frame's frame pointer
  	15  Chromium Framework!__ZNK15SkImage_GpuBase12onReadPixelsERK11SkImageInfoPvmiiN7SkImage11CachingHintE + 0x2ec
  	    rbp = 0x00007ffee89c0be0   rsp = 0x00007ffee89c0b40
  	    rip = 0x000000010d416f3c
  	    Found by: previous frame's frame pointer
  	16  Chromium Framework!__ZNK7SkImage10readPixelsERK8SkPixmapiiNS_11CachingHintE + 0x26
  	    rbp = 0x00007ffee89c0c00   rsp = 0x00007ffee89c0bf0
  	    rip = 0x000000010880a566
  	    Found by: previous frame's frame pointer
  	17  Chromium Framework!__ZN5blink16ImagePixelLockerC2E5sk_spIK7SkImageE11SkAlphaType11SkColorType + 0x256
  	    rbp = 0x00007ffee89c0db0   rsp = 0x00007ffee89c0c10
  	    rip = 0x00000001110f5686
  	    Found by: previous frame's frame pointer
  	18  Chromium Framework!__ZN4base8internal19OptionalStorageBaseIN5blink16ImagePixelLockerELb0EE4InitIJ5sk_spI7SkImageE11SkAlphaType11SkColorTypeEEEvDpOT_ + 0x87
  	    rbp = 0x00007ffee89c0f20   rsp = 0x00007ffee89c0dc0
  	    rip = 0x000000011108cfd7
  	    Found by: previous frame's frame pointer
  	19  Chromium Framework!__ZN5blink20WebGLImageConversion14ImageExtractor12ExtractImageEbb + 0x507
  	    rbp = 0x00007ffee89c10c0   rsp = 0x00007ffee89c0f30
  	    rip = 0x000000011106ee77
  	    Found by: previous frame's frame pointer
  	20  Chromium Framework!__ZN5blink25WebGLRenderingContextBase12TexImageImplENS0_18TexImageFunctionIDEjiiiiijjPNS_5ImageENS_20WebGLImageConversion18ImageHtmlDomSourceEbbRKNS_7IntRectEii + 0x1a4
  	    rbp = 0x00007ffee89c12f0   rsp = 0x00007ffee89c10d0
  	    rip = 0x0000000112ecb8f4
  	    Found by: previous frame's frame pointer
  	21  Chromium Framework!__ZN5blink25WebGLRenderingContextBase40TexImageHelperCanvasRenderingContextHostEPKNS_14SecurityOriginENS0_18TexImageFunctionIDEjiijjiiiPNS_26CanvasRenderingContextHostERKNS_7IntRectEiiRNS_14ExceptionStateE + 0x561
  	    rbp = 0x00007ffee89c1540   rsp = 0x00007ffee89c1300
  	    rip = 0x0000000112ecf151
  	    Found by: previous frame's frame pointer
  	22  Chromium Framework!__ZN5blink26WebGL2RenderingContextBase13texSubImage3DEPNS_16ExecutionContextEjiiiiiiijjPNS_26CanvasRenderingContextHostERNS_14ExceptionStateE + 0x102
  	    rbp = 0x00007ffee89c1620   rsp = 0x00007ffee89c1550
  	    rip = 0x0000000112e804b2
  	    Found by: previous frame's frame pointer
  	23  Chromium Framework!__ZN5blink36webgl2_rendering_context_v8_internalL19TexSubImage3DMethodERKN2v820FunctionCallbackInfoINS1_5ValueEEE + 0x1d93
  	    rbp = 0x00007ffee89c1720   rsp = 0x00007ffee89c1630
  	    rip = 0x0000000112784793
  	    Found by: previous frame's frame pointer
  	24  Chromium Framework!__ZN2v88internal25FunctionCallbackArguments4CallENS0_15CallHandlerInfoE + 0x3dd
  	    rbp = 0x00007ffee89c17d0   rsp = 0x00007ffee89c1730
  	    rip = 0x000000010aebaddd
  	    Found by: previous frame's frame pointer


We should try marking this test flaky on this platform.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f2ccb8906079b44ecaabd6b0fb6c683cc0a06624

commit f2ccb8906079b44ecaabd6b0fb6c683cc0a06624
Author: Kenneth Russell <kbr@chromium.org>
Date: Fri Jan 18 01:21:45 2019

Suppress two Mac NVIDIA WebGL conformance flakes.

  conformance/ogles/GL/exp2/exp2_001_to_008.html
  conformance2/textures/canvas/tex-3d-rg16f-rg-float.html

Bug:  922517 , 923080
No-Try: True
Change-Id: Icc604bd2af53b7ac2b447d05b2bd8507e05249fc
Tbr: senorblanco@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/1417839
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Stephen White <senorblanco@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623937}
[modify] https://crrev.com/f2ccb8906079b44ecaabd6b0fb6c683cc0a06624/content/test/gpu/gpu_tests/webgl2_conformance_expectations.py
[modify] https://crrev.com/f2ccb8906079b44ecaabd6b0fb6c683cc0a06624/content/test/gpu/gpu_tests/webgl_conformance_expectations.py

Looking green now.