Adjust paint invalidation rects to match the actual radius of rastered blur.

third_party/WebKit/Source/core/style/ShadowData.h

Index: third_party/WebKit/Source/core/style/ShadowData.h
diff --git a/third_party/WebKit/Source/core/style/ShadowData.h b/third_party/WebKit/Source/core/style/ShadowData.h
index fa404f7c812f5ec079ac6fd40638640e469c83a1..f68a4bd4cd7f67519d5bcf2f4c47f05abcb9c039 100644
--- a/third_party/WebKit/Source/core/style/ShadowData.h
+++ b/third_party/WebKit/Source/core/style/ShadowData.h
@@ -28,6 +28,7 @@
 #include "core/css/StyleColor.h"
 #include "platform/geometry/FloatPoint.h"
 #include "platform/geometry/FloatRectOutsets.h"
+#include "platform/graphics/skia/SkiaUtils.h"
 
 namespace blink {
 
@@ -63,7 +64,9 @@ public:
     // shadow.
     FloatRectOutsets rectOutsets() const
     {
-        float blurAndSpread = blur() + spread();
+        // 3 * skBlurRadiusToSigma(blur()) is how Skia implements the radius of a blur. See also
+        // https://crbug.com/624175.
+        float blurAndSpread = ceil(3 * skBlurRadiusToSigma(blur())) + spread();
         return FloatRectOutsets(
             blurAndSpread - y() /* top */,
             blurAndSpread + x() /* right */,