Clamp negative sigma to 0 in SkDropShadowImageFilter.

src/effects/SkDropShadowImageFilter.cpp

Index: src/effects/SkDropShadowImageFilter.cpp
diff --git a/src/effects/SkDropShadowImageFilter.cpp b/src/effects/SkDropShadowImageFilter.cpp
index 87dfa92f9bf3bf28469faa827e4aa1b905afb542..7f5f695a8e15d575a223bf94699f5a34b9154873 100644
--- a/src/effects/SkDropShadowImageFilter.cpp
+++ b/src/effects/SkDropShadowImageFilter.cpp
@@ -79,6 +79,8 @@ bool SkDropShadowImageFilter::onFilterImage(Proxy* proxy, const SkBitmap& source
 
     SkVector sigma, localSigma = SkVector::Make(fSigmaX, fSigmaY);
     matrix.mapVectors(&sigma, &localSigma, 1);
+    sigma.fX = SkMaxScalar(0, sigma.fX);
+    sigma.fY = SkMaxScalar(0, sigma.fY);
     SkAutoTUnref<SkImageFilter> blurFilter(new SkBlurImageFilter(sigma.fX, sigma.fY));
     SkAutoTUnref<SkColorFilter> colorFilter(SkColorFilter::CreateModeFilter(fColor, SkXfermode::kSrcIn_Mode));
     SkPaint paint;