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 8f9c2f027b6bacdd704cc19da08cc189e6fa51de..df67acd9ad961b9040140d68366377e67309342b 100644
--- a/cc/output/filter_operation.h
+++ b/cc/output/filter_operation.h
@@ -233,6 +233,10 @@ 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);