Revert of Temporarily skip ContextLost.WebGLContextLostFromGPUProcessExit on...

Revert of Temporarily skip ContextLost.WebGLContextLostFromGPUProcessExit on Mac OS. (patchset #1 id:1 of https://codereview.chromium.org/548583002/)

Reason for revert:
Scheduling for compositor tasks has been reverted so we can re-enable this test while we figure out the right way forward.

Original issue's description:
> Temporarily skip ContextLost.WebGLContextLostFromGPUProcessExit on Mac OS.
>
> Causing intermittent tab crashes, blocking CQ.
>
> BUG=411372
> TBR=skyostil@chromium.org
> NOTRY=true
>
> Committed: https://chromium.googlesource.com/chromium/src/+/ede18e2c3d7802fc37d51331a9c67e7960262677

TBR=kbr@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=411372

Review URL: https://codereview.chromium.org/559463002

Cr-Commit-Position: refs/heads/master@{#293910}

content/test/gpu/gpu_tests/context_lost_expectations.py

@@ -26,7 +26,3 @@ class ContextLostExpectations(test_expectations.TestExpectations):
 
     self.Fail('GpuCrash.GPUProcessCrashesExactlyOnce',
               ['win'], bug=392891)
-
-    # Skipping because of assertion failure / tab crash
-    self.Skip('ContextLost.WebGLContextLostFromGPUProcessExit',
-              ['mac'], bug=411372)