Consolidate FilterOperation and FilterEffect mapRect implementations

third_party/WebKit/Source/platform/graphics/filters/FEDropShadow.h

Index: third_party/WebKit/Source/platform/graphics/filters/FEDropShadow.h
diff --git a/third_party/WebKit/Source/platform/graphics/filters/FEDropShadow.h b/third_party/WebKit/Source/platform/graphics/filters/FEDropShadow.h
index cf4bd7abb6100ded98aa9594fa6ddee85573dec0..5cc47519e2c1550bbbf7c6b9e941223e220d2e3f 100644
--- a/third_party/WebKit/Source/platform/graphics/filters/FEDropShadow.h
+++ b/third_party/WebKit/Source/platform/graphics/filters/FEDropShadow.h
@@ -31,6 +31,12 @@ class PLATFORM_EXPORT FEDropShadow final : public FilterEffect {
   static FEDropShadow*
   create(Filter*, float, float, float, float, const Color&, float);
 
+  // Compute which destination area will be affected when applying a drop
+  // shadow effect with |stdDeviation| and |offset| to an area |rect|.
+  static FloatRect mapEffect(const FloatSize& stdDeviation,
+                             const FloatPoint& offset,
+                             const FloatRect&);
+
   void setShadowColor(const Color& color) { m_shadowColor = color; }
   void setShadowOpacity(float opacity) { m_shadowOpacity = opacity; }