Mark Pixel_ScissorTestWithPreserveDrawingBuffer flaky on macOS.

content/test/gpu/gpu_tests/pixel_expectations.py

Index: content/test/gpu/gpu_tests/pixel_expectations.py
diff --git a/content/test/gpu/gpu_tests/pixel_expectations.py b/content/test/gpu/gpu_tests/pixel_expectations.py
index a39e58e1f3761260367db7609f67d2e1695adf01..18f33a5be83715fbe083d77ce482013041d05ebd 100644
--- a/content/test/gpu/gpu_tests/pixel_expectations.py
+++ b/content/test/gpu/gpu_tests/pixel_expectations.py
@@ -37,6 +37,11 @@ class PixelExpectations(GpuTestExpectations):
     # TODO(kbr): flakily timing out on this configuration.
     self.Flaky('*', ['linux', 'intel', 'debug'], bug=648369)
 
+    # Flaky for unknown reasons only on macOS. Not planning to investigate
+    # further.
+    self.Flaky('Pixel_ScissorTestWithPreserveDrawingBuffer', ['mac'],
+               bug=660461)
+
     # TODO(zakerinasab): Check / generate reference images. Remove the Fail
     # lines after fixing 657946 and comment out the Skip line.
     self.Fail('Pixel_CanvasDisplayLinearRGBAccelerated2D', bug=657946)