The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/fd383c4428d3a4e1041425214f001661dca8d173

commit fd383c4428d3a4e1041425214f001661dca8d173
Author: qiankun.miao <qiankun.miao@intel.com>
Date: Fri Jul 15 03:26:30 2016

Discard correct drawing buffer

This partly revert https://codereview.chromium.org/1950053003. On
platform supported GL_EXT_discard_framebuffer extension, it causes a
WebGL conformance test regression:
conformance/state/state-uneffected-after-compositing.html

The root cause is current bound fbo will be discarded other than the
default fbo. We should discard default fbo correctly.

BUG= 628472 

Review-Url: https://codereview.chromium.org/2145093003
Cr-Commit-Position: refs/heads/master@{#405689}

[modify] https://crrev.com/fd383c4428d3a4e1041425214f001661dca8d173/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp

This is a very interesting case, as we tried many combinations of OS and driver, and switched between GL and GLES mode, but only Linux+IntelMesa+GLES (with --use-gl=egl option) failed the case. After investigation, it turns out to be a bug inside Chromium, rather than a driver issue as we previously suspected. The issue is only triggered with extension "discard_framebuffer" valid. Below are the reasons why other combinations couldn't trigger the issue:

Linux+NVIDIA+GL: no support of extension "discard_framebuffer"
Linux+NVIDIA+GLES: has support of extension "discard_framebuffer", but it's disabled due to https://bugs.chromium.org/p/chromium/issues/detail?id=601753.
Linux+IntelMesa+GL: no support of extension "discard_framebuffer"
ChromeOS+IntelMesa+GLES: has support of extension "discard_framebuffer", but it's disabled due to https://bugs.chromium.org/p/chromium/issues/detail?id=301988 (a Mali issue, and I will follow up to see if Intel GEN GPU can work well).

So the conclusion is this is an ES extension, but for NVIDIA ES and ChromeOS (implying ES), the extension was disabled for different reasons. 

Ha, you won the lottery :)

Thanks for the thorough investigation!

Your change meets the bar and is auto-approved for M53 (branch: 2785)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/87afbee2e2f123fc576d4274a563f2a628189501

commit 87afbee2e2f123fc576d4274a563f2a628189501
Author: erikchen <erikchen@chromium.org>
Date: Wed Aug 24 17:12:40 2016

[Merge to 2785] Discard correct drawing buffer

> This partly revert https://codereview.chromium.org/1950053003. On
> platform supported GL_EXT_discard_framebuffer extension, it causes a
> WebGL conformance test regression:
> conformance/state/state-uneffected-after-compositing.html
>
> The root cause is current bound fbo will be discarded other than the
> default fbo. We should discard default fbo correctly.
>
> BUG= 628472 
>
> Review-Url: https://codereview.chromium.org/2145093003
> Cr-Commit-Position: refs/heads/master@{#405689}
> (cherry picked from commit fd383c4428d3a4e1041425214f001661dca8d173)

Review URL: https://codereview.chromium.org/2267423004 .

Cr-Commit-Position: refs/branch-heads/2785@{#738}
Cr-Branched-From: 68623971be0cfc492a2cb0427d7f478e7b214c24-refs/heads/master@{#403382}

[modify] https://crrev.com/87afbee2e2f123fc576d4274a563f2a628189501/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp

Verified the fix on Ken's linux Machine(Thank you Ken) with Chrome M53(53.0.2785.89). 

Steps followed : 
1.  open chrome with command line: --enable-unsafe-es3-apis --use-gl=egl on Linux
2. open https://www.khronos.org/registry/webgl/sdk/tests/conformance/state/state-uneffected-after-compositing.html?webglVersion=1

Observed behavior :
All tests passed.