maps_pixel_test (gpu_tests): mark Maps_maps_004 as flaky on Win10 NVidia.

R=kbr@chromium.org
BUG=670098
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_optional_gpu_tests_rel;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

Review-Url: https://codereview.chromium.org/2852453002
Cr-Commit-Position: refs/heads/master@{#467960}

content/test/gpu/gpu_tests/maps_expectations.py

@@ -11,4 +11,4 @@ class MapsExpectations(GpuTestExpectations):
     # Sample Usage:
     # self.Fail('Maps.maps_001',
     #     ['mac', 'amd', ('nvidia', 0x1234)], bug=123)
-    pass
+    self.Flaky('Maps_maps_004', ['win10', 'nvidia'], bug=670098)