trace_tests: Mark some windows tests as flaky.

BUG=638744
R=kbr@chromium.org
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
NOTRY=true

Review-Url: https://codereview.chromium.org/2276123002
Cr-Commit-Position: refs/heads/master@{#414238}

content/test/gpu/gpu_tests/trace_test_expectations.py

@@ -11,7 +11,10 @@ class TraceTestExpectations(GpuTestExpectations):
     # Sample Usage:
     # self.Fail('TraceTest.Canvas2DRedBox',
     #     ['mac', 'amd', ('nvidia', 0x1234)], bug=123)
-    pass
+    self.Flaky('TraceTest.2DCanvasWebGL', ['win'], bug=638744)
+    self.Flaky('TraceTest.SolidColorBackground', ['win'], bug=638744)
+    self.Flaky('TraceTest.WebGLGreenTriangle.AA.NoAlpha', ['win'], bug=638744)
+    self.Flaky('TraceTest.WebGLGreenTriangle.NoAA.Alpha', ['win'], bug=638744)
 
 
 class DeviceTraceTestExpectations(GpuTestExpectations):