Fix ExternalViewportRectForPrioritizingTiles flakiness on windows

cc/layers/picture_layer_impl_unittest.cc

Index: cc/layers/picture_layer_impl_unittest.cc
diff --git a/cc/layers/picture_layer_impl_unittest.cc b/cc/layers/picture_layer_impl_unittest.cc
index 8136dcdaedcad9da624d78c241e7ef0b64cfc0ba..79da519ae097b24c3a35a44551a2b5925a5ef30e 100644
--- a/cc/layers/picture_layer_impl_unittest.cc
+++ b/cc/layers/picture_layer_impl_unittest.cc
@@ -303,9 +303,9 @@ TEST_F(PictureLayerImplTest, CloneNoInvalidation) {
     VerifyAllTilesExistAndHavePile(tilings->tiling_at(i), active_pile.get());
 }
 
-TEST_F(PictureLayerImplTest,
-       DISABLED_ExternalViewportRectForPrioritizingTiles) {
+TEST_F(PictureLayerImplTest, ExternalViewportRectForPrioritizingTiles) {
   base::TimeTicks time_ticks;
+  time_ticks += base::TimeDelta::FromMilliseconds(1);
   host_impl_.SetCurrentFrameTimeTicks(time_ticks);
   gfx::Size tile_size(100, 100);
   gfx::Size layer_bounds(400, 400);
@@ -321,6 +321,9 @@ TEST_F(PictureLayerImplTest,
   AddDefaultTilingsWithInvalidation(invalidation);
   SetupDrawPropertiesAndUpdateTiles(active_layer_, 1.f, 1.f, 1.f, 1.f, false);
 
+  time_ticks += base::TimeDelta::FromMilliseconds(200);
+  host_impl_.SetCurrentFrameTimeTicks(time_ticks);
+
   // Update tiles with viewport for tile priority as (0, 0, 100, 100) and the
   // identify transform for tile priority.
   bool resourceless_software_draw = false;