SkPictureShader should handle negative scaling gracefully.

src/core/SkPictureShader.cpp

Index: src/core/SkPictureShader.cpp
diff --git a/src/core/SkPictureShader.cpp b/src/core/SkPictureShader.cpp
index 5a1e61849df05b81afe0dcc2dddc227aa7ebc6b1..819a0d0ae71c0451cb2b206423e2e1c158fc00f4 100644
--- a/src/core/SkPictureShader.cpp
+++ b/src/core/SkPictureShader.cpp
@@ -157,7 +157,8 @@ SkShader* SkPictureShader::refBitmapShader(const SkMatrix& matrix, const SkMatri
         scale.set(SkScalarSqrt(m.getScaleX() * m.getScaleX() + m.getSkewX() * m.getSkewX()),
                   SkScalarSqrt(m.getScaleY() * m.getScaleY() + m.getSkewY() * m.getSkewY()));
     }
-    SkSize scaledSize = SkSize::Make(scale.x() * fTile.width(), scale.y() * fTile.height());
+    SkSize scaledSize = SkSize::Make(SkScalarAbs(scale.x() * fTile.width()),
+                                     SkScalarAbs(scale.y() * fTile.height()));
 
     // Clamp the tile size to about 16M pixels
     static const SkScalar kMaxTileArea = 4096 * 4096;