Set mac context_lost_tests as flaky

Three tests are flaking, mark them as flaky until root cause can be
investigated.

Bug: 965268, 1031095, 1031567
Change-Id: I8b0a85f1385250abca8264278ec2dcd97531dcba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1956017
Commit-Queue: kylechar <kylechar@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Auto-Submit: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#722668}

content/test/gpu/gpu_tests/test_expectations/context_lost_expectations.txt

@@ -42,3 +42,8 @@ crbug.com/611906 [ android qualcomm-adreno-(tm)-420 ] ContextLost_WebGLContextLo
 crbug.com/609629 [ android qualcomm-adreno-(tm)-420 ] GpuCrash_GPUProcessCrashesExactlyOncePerVisitToAboutGpuCrash [ Skip ]
 crbug.com/609629 [ android qualcomm-adreno-(tm)-420 ] ContextLost_WebGLContextLostFromGPUProcessExit [ Skip ]
 crbug.com/609629 [ android qualcomm-adreno-(tm)-420 ] ContextLost_WebGLContextLostInHiddenTab [ Skip ]
+
+# RetryOnFailure flaky mac test until root cause can be investigated.
+crbug.com/965268 [ mac ] ContextLost_WebGLUnblockedAfterUserInitiatedReload [ RetryOnFailure ]
+crbug.com/1031095 [ mac ] GpuCrash_GPUProcessCrashesExactlyOncePerVisitToAboutGpuCrash [ RetryOnFailure ]
+crbug.com/1031567 [ mac ] ContextLost_WebGLContextLostFromGPUProcessExit [ RetryOnFailure ]