[WebGL][Tests] Preserve Drawing buffer conformance test fails.

[kalyankondapally]

Steps to reproduce:
Run WebGL Conformance test locally:

content/test/gpu/run_gpu_test --browser=exact --browser-executable=out/Debug/chrome --extra-browser-args=--no-sandbox webgl_conformance --webgl-conformance-version=1.0.3 -v --page-filter=context-attribute-preserve-drawing-buffer

Expected result:
Test passed.

Actual result:
Test failed.

Dependency: #29.
Blocks: #143 .
SubTasks: None.
Works on Chromium-X11: Yes.

[kalyankondapally]

[ RUN ] WebglConformance.conformance_context_context_attribute_preserve_drawing_buffer
INFO:root:Running file://conformance/context/context-attribute-preserve-drawing-buffer.html
WARNING:root:file://conformance/context/context-attribute-preserve-drawing-buffer.html:
Traceback (most recent call last):
File "content/test/gpu/../../../tools/telemetry/telemetry/page/page_runner.py", line 442, in _RunPage
test.Run(finder_options, page, page_state.tab, results)
File "content/test/gpu/../../../tools/telemetry/telemetry/page/page_test.py", line 218, in Run
self._test_method(page, tab, results)
File "/home/kalyan/chromium/src/content/test/gpu/gpu_tests/webgl_conformance.py", line 66, in ValidatePage
raise page_test.Failure(_WebGLTestMessages(tab))
Failure: requestAnimationFrame
cancelAnimationFrame
Did not render ok with preserveDrawingBuffer false.

Traceback (most recent call last):
File "content/test/gpu/../../../tools/telemetry/telemetry/page/page_runner.py", line 442, in _RunPage
test.Run(finder_options, page, page_state.tab, results)
File "content/test/gpu/../../../tools/telemetry/telemetry/page/page_test.py", line 218, in Run
self._test_method(page, tab, results)
File "/home/kalyan/chromium/src/content/test/gpu/gpu_tests/webgl_conformance.py", line 66, in ValidatePage
raise page_test.Failure(_WebGLTestMessages(tab))
Failure: requestAnimationFrame
cancelAnimationFrame
Did not render ok with preserveDrawingBuffer false.

[ FAILED ] WebglConformance.conformance_context_context_attribute_preserve_drawing_buffer (548 ms)

[kalyankondapally]

Can't reproduce this anymore.