cc: Guard overdraw metrics behind the --trace-overdraw command-line flag.

cc/quad_culler_unittest.cc

Index: cc/quad_culler_unittest.cc
diff --git a/cc/quad_culler_unittest.cc b/cc/quad_culler_unittest.cc
index 90c27d0173bfce5f1daa2c9c64a1d38e9091c1eb..162a5fbf02d2adafeda45cd311a9dc417b41dae9 100644
--- a/cc/quad_culler_unittest.cc
+++ b/cc/quad_culler_unittest.cc
@@ -221,7 +221,7 @@ TEST(QuadCullerTest, verifyCullCenterTileNonIntegralSize1)
     appendQuads(quadList, sharedStateList, rootLayer.get(), it, occlusionTracker);
     EXPECT_EQ(quadList.size(), 2u);
 
-    EXPECT_NEAR(occlusionTracker.overdrawMetrics().pixelsDrawnOpaque(), 20000, 1);
+    EXPECT_NEAR(occlusionTracker.overdrawMetrics().pixelsDrawnOpaque(), 20363, 1);
     EXPECT_NEAR(occlusionTracker.overdrawMetrics().pixelsDrawnTranslucent(), 0, 1);
     EXPECT_NEAR(occlusionTracker.overdrawMetrics().pixelsCulledForDrawing(), 0, 1);
 }
@@ -249,7 +249,7 @@ TEST(QuadCullerTest, verifyCullCenterTileNonIntegralSize2)
     appendQuads(quadList, sharedStateList, rootLayer.get(), it, occlusionTracker);
     EXPECT_EQ(quadList.size(), 2u);
 
-    EXPECT_NEAR(occlusionTracker.overdrawMetrics().pixelsDrawnOpaque(), 20000, 1);
+    EXPECT_NEAR(occlusionTracker.overdrawMetrics().pixelsDrawnOpaque(), 19643, 1);
     EXPECT_NEAR(occlusionTracker.overdrawMetrics().pixelsDrawnTranslucent(), 0, 1);
     EXPECT_NEAR(occlusionTracker.overdrawMetrics().pixelsCulledForDrawing(), 0, 1);
 }