Do not map empty rect in FilterOperation.

If the input rect is empty, we do not want to map the rect to a
non-empty rect. Because it could cause problem, for example, in
damage_tracker to have a damaged region due to having pixel-moving
filters.

DamageTrackerTest.VerifyDamageForBackgroundBlurredChild

Bug: 745101
Test: DamageTrackerTest.VerifyDamageForImageFilter and
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I8719f5ad2e6da08b3a95df45c5194532c3bc5bd4
Reviewed-on: https://chromium-review.googlesource.com/587090
Commit-Queue: Tao Wu <wutao@chromium.org>
Reviewed-by: Ali Juma <ajuma@chromium.org>
Reviewed-by: Stephen White <senorblanco@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490580}

cc/trees/damage_tracker.cc

@@ -187,7 +187,7 @@ void DamageTracker::ComputeSurfaceDamage(RenderSurfaceImpl* render_surface) {
 
     gfx::Rect damage_rect;
     bool is_rect_valid = damage_for_this_update_.GetAsRect(&damage_rect);
-    if (is_rect_valid) {
+    if (is_rect_valid && !damage_rect.IsEmpty()) {
       damage_rect = render_surface->Filters().MapRect(
           damage_rect, render_surface->SurfaceScale().matrix());
       damage_for_this_update_ = DamageAccumulator();
@@ -331,9 +331,9 @@ void DamageTracker::ExpandDamageInsideRectWithFilters(
     const FilterOperations& filters) {
   gfx::Rect damage_rect;
   bool is_valid_rect = damage_for_this_update_.GetAsRect(&damage_rect);
-  // If the damage accumulated so far isn't a valid rect, then there is no point
-  // in trying to make it bigger.
-  if (!is_valid_rect)
+  // If the damage accumulated so far isn't a valid rect or empty, then there is
+  // no point in trying to make it bigger.
+  if (!is_valid_rect || damage_rect.IsEmpty())
     return;
 
   // Compute the pixels in the background of the surface that could be affected

cc/trees/damage_tracker_unittest.cc

@@ -610,6 +610,19 @@ TEST_F(DamageTrackerTest, VerifyDamageForImageFilter) {
 
   // gfx::Rect(0, 0, 1, 1), expanded by 6px for the 2px blur filter.
   EXPECT_EQ(gfx::Rect(-6, -6, 13, 13), child_damage_rect);
+
+  // CASE 2: No changes, so should not damage the surface.
+  ClearDamageForAllSurfaces(root);
+  EmulateDrawingOneFrame(root);
+
+  EXPECT_TRUE(GetRenderSurface(root)->damage_tracker()->GetDamageRectIfValid(
+      &root_damage_rect));
+  EXPECT_TRUE(GetRenderSurface(child)->damage_tracker()->GetDamageRectIfValid(
+      &child_damage_rect));
+
+  // Should not be expanded by the blur filter.
+  EXPECT_EQ(gfx::Rect(), root_damage_rect);
+  EXPECT_EQ(gfx::Rect(), child_damage_rect);
 }
 
 TEST_F(DamageTrackerTest, VerifyDamageForTransformedImageFilter) {
@@ -847,6 +860,26 @@ TEST_F(DamageTrackerTest, VerifyDamageForBackgroundBlurredChild) {
   // 100,100.
   expected_damage_rect = gfx::Rect(100, 100, 7, 7);
   EXPECT_EQ(expected_damage_rect.ToString(), root_damage_rect.ToString());
+
+  // CASE 7: No changes, so should not damage the surface.
+  ClearDamageForAllSurfaces(root);
+  // We want to make sure that the background filter doesn't cause empty damage
+  // to get expanded. We position child1 so that an expansion of the empty rect
+  // would have non-empty intersection with child1 in its target space (root
+  // space).
+  child1->SetPosition(gfx::PointF());
+  root->layer_tree_impl()->property_trees()->needs_rebuild = true;
+  EmulateDrawingOneFrame(root);
+
+  gfx::Rect child_damage_rect;
+  EXPECT_TRUE(GetRenderSurface(root)->damage_tracker()->GetDamageRectIfValid(
+      &root_damage_rect));
+  EXPECT_TRUE(GetRenderSurface(child1)->damage_tracker()->GetDamageRectIfValid(
+      &child_damage_rect));
+
+  // Should not be expanded by the blur filter.
+  EXPECT_EQ(gfx::Rect(), root_damage_rect);
+  EXPECT_EQ(gfx::Rect(), child_damage_rect);
 }
 
 TEST_F(DamageTrackerTest, VerifyDamageForAddingAndRemovingLayer) {