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

cc/output/filter_operation.h

Index: cc/output/filter_operation.h
diff --git a/cc/output/filter_operation.h b/cc/output/filter_operation.h
index c729e0b173bfa9f26a8f34ad076db0772812a4f9..f813e0b001d6da9c7e8d53c85dc8d3f7b3ae7ab6 100644
--- a/cc/output/filter_operation.h
+++ b/cc/output/filter_operation.h
@@ -235,6 +235,10 @@ class CC_EXPORT FilterOperation {
   // by pixels in the source rect.
   gfx::Rect MapRect(const gfx::Rect& rect, const SkMatrix& matrix) const;
 
+  // Maps "backward" to determine which pixels in the source affect the pixels
+  // in the destination rect.
+  gfx::Rect MapRectReverse(const gfx::Rect& rect, const SkMatrix& matrix) const;
+
  private:
   FilterOperation(FilterType type, float amount);