Dirty rects always contain full tiles with delegated rendering.

cc/layers/tiled_layer.cc

Index: cc/layers/tiled_layer.cc
diff --git a/cc/layers/tiled_layer.cc b/cc/layers/tiled_layer.cc
index a65067203783563902edbc06ee677aabbad7f258..24baef526eb67025dbfc24e929aa7db4eb8709a5 100644
--- a/cc/layers/tiled_layer.cc
+++ b/cc/layers/tiled_layer.cc
@@ -334,8 +334,10 @@ bool TiledLayer::UpdateTiles(int left,
     return false;
   }
 
-  gfx::Rect paint_rect =
-      MarkTilesForUpdate(left, top, right, bottom, ignore_occlusions);
+  gfx::Rect update_rect;
+  gfx::Rect paint_rect;
+  MarkTilesForUpdate(
+    &update_rect, &paint_rect, left, top, right, bottom, ignore_occlusions);
 
   if (occlusion)
     occlusion->overdraw_metrics()->DidPaint(paint_rect);
@@ -345,7 +347,7 @@ bool TiledLayer::UpdateTiles(int left,
 
   *updated = true;
   UpdateTileTextures(
-      paint_rect, left, top, right, bottom, queue, occlusion);
+      update_rect, paint_rect, left, top, right, bottom, queue, occlusion);
   return true;
 }
 
@@ -422,12 +424,13 @@ bool TiledLayer::HaveTexturesForTiles(int left,
   return true;
 }
 
-gfx::Rect TiledLayer::MarkTilesForUpdate(int left,
-                                         int top,
-                                         int right,
-                                         int bottom,
-                                         bool ignore_occlusions) {
-  gfx::Rect paint_rect;
+void TiledLayer::MarkTilesForUpdate(gfx::Rect* update_rect,
+                                    gfx::Rect* paint_rect,
+                                    int left,
+                                    int top,
+                                    int right,
+                                    int bottom,
+                                    bool ignore_occlusions) {
   for (int j = top; j <= bottom; ++j) {
     for (int i = left; i <= right; ++i) {
       UpdatableTile* tile = TileAt(i, j);
@@ -437,6 +440,10 @@ gfx::Rect TiledLayer::MarkTilesForUpdate(int left,
         continue;
       if (tile->occluded && !ignore_occlusions)
         continue;
+
+      // Prepare update rect from original dirty rects.
+      update_rect->Union(tile->dirty_rect);
+
       // TODO(reveman): Decide if partial update should be allowed based on cost
       // of update. https://bugs.webkit.org/show_bug.cgi?id=77376
       if (tile->is_dirty() &&
@@ -455,14 +462,14 @@ gfx::Rect TiledLayer::MarkTilesForUpdate(int left,
         }
       }
 
-      paint_rect.Union(tile->dirty_rect);
+      paint_rect->Union(tile->dirty_rect);
       tile->MarkForUpdate();
     }
   }
-  return paint_rect;
 }
 
-void TiledLayer::UpdateTileTextures(gfx::Rect paint_rect,
+void TiledLayer::UpdateTileTextures(gfx::Rect update_rect,
+                                    gfx::Rect paint_rect,
                                     int left,
                                     int top,
                                     int right,
@@ -477,7 +484,7 @@ void TiledLayer::UpdateTileTextures(gfx::Rect paint_rect,
   float height_scale =
       paint_properties().bounds.height() /
       static_cast<float>(content_bounds().height());
-  update_rect_ = gfx::ScaleRect(paint_rect, width_scale, height_scale);
+  update_rect_ = gfx::ScaleRect(update_rect, width_scale, height_scale);
 
   // Calling PrepareToUpdate() calls into WebKit to paint, which may have the
   // side effect of disabling compositing, which causes our reference to the