Draw correct repaint regions instead of a conservative bounding box

Previously, the update_rect() of the LayerImpl was used for the repaint
regions in the DebugRectHistory; now the base LayerImpl class defines a
virtual method giving back an invalidation Region that is overwritten by
PictureLayerImpl to use the already existent invalidations_ Region.
Regions give a more tighter bound on the invalidations than the
single conservative rect that was used before on a per-layer basis.

BUG=424682

Review URL: https://codereview.chromium.org/958843004

Cr-Commit-Position: refs/heads/master@{#319094}

AUTHORS

@@ -109,6 +109,7 @@ Daniel Carvalho Liedke <dliedke@gmail.com>
 Daniel Imms <daniimms@amazon.com>
 Daniel Johnson <danielj41@gmail.com>
 Daniel Nishi <dhnishi@gmail.com>
+Daniel Platz <daplatz@googlemail.com>
 Daniel Shaulov <dshaulov@ptc.com>
 Daniel Trebbien <dtrebbien@gmail.com>
 Darshini KN <kn.darshini@samsung.com>

cc/debug/debug_rect_history.cc

@@ -68,17 +68,21 @@ void DebugRectHistory::SavePaintRects(LayerImpl* layer) {
   // not. Therefore we traverse recursively over all layers, not just the render
   // surface list.
 
-  if (!layer->update_rect().IsEmpty() && layer->DrawsContent()) {
+  Region invalidation_region = layer->GetInvalidationRegion();
+  if (!invalidation_region.IsEmpty() && layer->DrawsContent()) {
     float width_scale = layer->content_bounds().width() /
                         static_cast<float>(layer->bounds().width());
     float height_scale = layer->content_bounds().height() /
                          static_cast<float>(layer->bounds().height());
-    gfx::Rect update_content_rect = gfx::ScaleToEnclosingRect(
-        layer->update_rect(), width_scale, height_scale);
-    debug_rects_.push_back(
-        DebugRect(PAINT_RECT_TYPE,
-                  MathUtil::MapEnclosingClippedRect(
-                      layer->screen_space_transform(), update_content_rect)));
+
+    for (Region::Iterator it(invalidation_region); it.has_rect(); it.next()) {
+      gfx::Rect update_content_rect =
+          gfx::ScaleToEnclosingRect(it.rect(), width_scale, height_scale);
+      debug_rects_.push_back(
+          DebugRect(PAINT_RECT_TYPE,
+                    MathUtil::MapEnclosingClippedRect(
+                        layer->screen_space_transform(), update_content_rect)));
+    }
   }
 
   for (unsigned i = 0; i < layer->children().size(); ++i)

cc/layers/layer_impl.cc

@@ -1590,4 +1590,8 @@ void LayerImpl::SetHasRenderSurface(bool should_have_render_surface) {
   render_surface_.reset();
 }
 
+Region LayerImpl::GetInvalidationRegion() {
+  return Region(update_rect_);
+}
+
 }  // namespace cc

cc/layers/layer_impl.h

@@ -596,6 +596,10 @@ class CC_EXPORT LayerImpl : public LayerAnimationValueObserver,
 
   SyncedScrollOffset* synced_scroll_offset() { return scroll_offset_.get(); }
 
+  // Get the correct invalidation region instead of conservative Rect
+  // for layers that provide it.
+  virtual Region GetInvalidationRegion();
+
  protected:
   LayerImpl(LayerTreeImpl* layer_impl,
             int id,

cc/layers/picture_layer_impl.cc

@@ -605,6 +605,15 @@ skia::RefPtr<SkPicture> PictureLayerImpl::GetPicture() {
   return raster_source_->GetFlattenedPicture();
 }
 
+Region PictureLayerImpl::GetInvalidationRegion() {
+  // |invalidation_| gives the invalidation contained in the source frame, but
+  // is not cleared after drawing from the layer. However, update_rect() is
+  // cleared once the invalidation is drawn, which is useful for debugging
+  // visualizations. This method intersects the two to give a more exact
+  // representation of what was invalidated that is cleared after drawing.
+  return IntersectRegions(invalidation_, update_rect());
+}
+
 scoped_refptr<Tile> PictureLayerImpl::CreateTile(
     float contents_scale,
     const gfx::Rect& content_rect) {

cc/layers/picture_layer_impl.h

@@ -61,6 +61,7 @@ class CC_EXPORT PictureLayerImpl
   void ReleaseResources() override;
   void RecreateResources() override;
   skia::RefPtr<SkPicture> GetPicture() override;
+  Region GetInvalidationRegion() override;
 
   // PictureLayerTilingClient overrides.
   scoped_refptr<Tile> CreateTile(float contents_scale,