cc: Implement and use FilterOperations::MapRectReverse for mapping backdrop bounding box.

cc/output/filter_operation.cc

Index: cc/output/filter_operation.cc
diff --git a/cc/output/filter_operation.cc b/cc/output/filter_operation.cc
index 78588748ba59d63c5d68bd35e1df56cba76f0d22..c7eae994fc66150a06ad6c8903f52c0956578d95 100644
--- a/cc/output/filter_operation.cc
+++ b/cc/output/filter_operation.cc
@@ -352,4 +352,34 @@ gfx::Rect FilterOperation::MapRect(const gfx::Rect& rect,
   }
 }
 
+gfx::Rect FilterOperation::MapRectReverse(const gfx::Rect& rect,
+                                          const SkMatrix& matrix) const {
+  switch (type_) {
+    case FilterOperation::BLUR: {
+      SkVector spread = MapStdDeviation(amount(), matrix);
+      gfx::Rect result = rect;
+      result.Inset(-spread.x(), -spread.y(), -spread.x(), -spread.y());
+      return result;
+    }
+    case FilterOperation::DROP_SHADOW: {
+      SkVector spread = MapStdDeviation(amount(), matrix);
+      gfx::Rect result = rect;
+      result.Inset(-spread.x(), -spread.y(), -spread.x(), -spread.y());
+      result -= drop_shadow_offset().OffsetFromOrigin();
+      result.Union(rect);
+      return result;
+    }
+    case FilterOperation::REFERENCE: {
+      if (!image_filter())
+        return rect;
+      SkIRect in_rect = gfx::RectToSkIRect(rect);
+      SkIRect out_rect = image_filter()->filterBounds(
+          in_rect, matrix, SkImageFilter::kReverse_MapDirection);
+      return gfx::SkIRectToRect(out_rect);
+    }
+    default:
+      return rect;
+  }
+}
+
 }  // namespace cc