Dynamically create stencil buffer when needed.

src/effects/SkAlphaThresholdFilter.cpp

Index: src/effects/SkAlphaThresholdFilter.cpp
diff --git a/src/effects/SkAlphaThresholdFilter.cpp b/src/effects/SkAlphaThresholdFilter.cpp
index a5b564f57bb51cf3029ac3341ce6932f85ed5d9c..f78c9c0b8cd7374c06213707dbb6d14ebbe4e741 100644
--- a/src/effects/SkAlphaThresholdFilter.cpp
+++ b/src/effects/SkAlphaThresholdFilter.cpp
@@ -273,7 +273,7 @@ bool SkAlphaThresholdFilterImpl::asFragmentProcessor(GrFragmentProcessor** fp,
         } else {
             maskDesc.fConfig = kRGBA_8888_GrPixelConfig;
         }
-        maskDesc.fFlags = kRenderTarget_GrSurfaceFlag | kNoStencil_GrSurfaceFlag;
+        maskDesc.fFlags = kRenderTarget_GrSurfaceFlag;
         // Add one pixel of border to ensure that clamp mode will be all zeros
         // the outside.
         maskDesc.fWidth = texture->width();