Switch GrTextureToYUVPlanes over to newDrawContext calls

include/gpu/GrContext.h

Index: include/gpu/GrContext.h
diff --git a/include/gpu/GrContext.h b/include/gpu/GrContext.h
index 44dfda65ee0eaef5fe4bf1047958291fbaa5e126..46612b110deb58d849883d96744b82bf97d1cf85 100644
--- a/include/gpu/GrContext.h
+++ b/include/gpu/GrContext.h
@@ -196,6 +196,8 @@ public:
     /**
      * Create both a GrRenderTarget and a matching GrDrawContext to wrap it.
      * The created GrRenderTarget will always be budgeted.
+     * We guarantee that "asTexture" will succeed for drawContexts created
+     * via this entry point.
      */
     sk_sp<GrDrawContext> newDrawContext(SkBackingFit fit, 
                                         int width, int height,