I traced the code of DrawingBuffer::TransferToStaticBitmapImage() and came all the way to this issue. This particular function causes two OffscreenCanvas API to
result in flaky rendering output as in https://bugs.chromium.org/p/chromium/issues/detail?id=807742 and https://bugs.chromium.org/p/chromium/issues/detail?id=735228.

VRDisplay is yet another API that uses this function but it seems to have  no problems with this. I find that the major difference is that the function that uses DrawingBuffer::TransferToStaticBitmapImage() in VRDisplay has made use of
the release call back in a meaningful way:

https://cs.chromium.org/chromium/src/third_party/WebKit/Source/platform/graphics/gpu/XRFrameTransport.h?l=75

I do not know whether this is related to the flakiness. Even if they are not related, I think this is an area that we need to improve for OffscreenCanvas.commit() in webgl rendering context.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4806cee353459fd73ca4c7522701666488e5efc4

commit 4806cee353459fd73ca4c7522701666488e5efc4
Author: xlai <xlai@chromium.org>
Date: Fri Apr 13 20:11:13 2018

Add image release callback to Webgl commit() in OffscreenCanvas

DrawingBuffer::TransferToStaticBitmapImage() is the common function shared by
commit() and transferToImageBitmap() in OffscreenCanvas WebGL, as well as the
VR display frame submission. The two OffscreenCanvas APIs miss a release
callback; as a result, release_callback populated by image extraction function
PrepareTransferableResourceInternal is invoked very early with a "lost
resource" signal. On the other hand, VR display frame submission has correctly
implemented a release callback that is releasing previous frame whenever there
is a new frame coming in.

This is a very likely reason to contribute to the flakiness of these two
OffscreenCanvas APIs, while the VR display is working well.

This CL adds a release callback to OffscreenCanvas commit(). I will follow-up
with another CL that adds release callback to OffscreenCanvas
transferToImageBitmap() and another CL that enables the relevant gpu
pixel tests.



Bug:  809227 ,  735228 
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: Ia6e2c613de416f1e6e2a0f1c833d9e82e47fed4f
Reviewed-on: https://chromium-review.googlesource.com/1010466
Reviewed-by: Justin Novosad <junov@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Commit-Queue: Olivia Lai <xlai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550740}
[modify] https://crrev.com/4806cee353459fd73ca4c7522701666488e5efc4/third_party/blink/renderer/modules/webgl/webgl_rendering_context_base.cc
[modify] https://crrev.com/4806cee353459fd73ca4c7522701666488e5efc4/third_party/blink/renderer/platform/graphics/gpu/drawing_buffer.cc
[modify] https://crrev.com/4806cee353459fd73ca4c7522701666488e5efc4/third_party/blink/renderer/platform/graphics/gpu/drawing_buffer.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4806cee353459fd73ca4c7522701666488e5efc4

commit 4806cee353459fd73ca4c7522701666488e5efc4
Author: xlai <xlai@chromium.org>
Date: Fri Apr 13 20:11:13 2018

Add image release callback to Webgl commit() in OffscreenCanvas

DrawingBuffer::TransferToStaticBitmapImage() is the common function shared by
commit() and transferToImageBitmap() in OffscreenCanvas WebGL, as well as the
VR display frame submission. The two OffscreenCanvas APIs miss a release
callback; as a result, release_callback populated by image extraction function
PrepareTransferableResourceInternal is invoked very early with a "lost
resource" signal. On the other hand, VR display frame submission has correctly
implemented a release callback that is releasing previous frame whenever there
is a new frame coming in.

This is a very likely reason to contribute to the flakiness of these two
OffscreenCanvas APIs, while the VR display is working well.

This CL adds a release callback to OffscreenCanvas commit(). I will follow-up
with another CL that adds release callback to OffscreenCanvas
transferToImageBitmap() and another CL that enables the relevant gpu
pixel tests.



Bug:  809227 ,  735228 
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: Ia6e2c613de416f1e6e2a0f1c833d9e82e47fed4f
Reviewed-on: https://chromium-review.googlesource.com/1010466
Reviewed-by: Justin Novosad <junov@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Commit-Queue: Olivia Lai <xlai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550740}
[modify] https://crrev.com/4806cee353459fd73ca4c7522701666488e5efc4/third_party/blink/renderer/modules/webgl/webgl_rendering_context_base.cc
[modify] https://crrev.com/4806cee353459fd73ca4c7522701666488e5efc4/third_party/blink/renderer/platform/graphics/gpu/drawing_buffer.cc
[modify] https://crrev.com/4806cee353459fd73ca4c7522701666488e5efc4/third_party/blink/renderer/platform/graphics/gpu/drawing_buffer.h

This got resolved indirectly with the CanvasResource refactoring work.