Add asDeferredTexture & asDeferredRenderTarget helpers to GrSurfaceContext

These are proving useful in the read/write-Pixels migration

Change-Id: I297f31968362d205977b769808320b1dc06249df
Reviewed-on: https://skia-review.googlesource.com/5936
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
diff --git a/include/gpu/GrTextureContext.h b/include/gpu/GrTextureContext.h
index da71c07..28fe63d 100644
--- a/include/gpu/GrTextureContext.h
+++ b/include/gpu/GrTextureContext.h
@@ -29,6 +29,10 @@
 
     bool copySurface(GrSurface* src, const SkIRect& srcRect, const SkIPoint& dstPoint) override;
 
+    GrSurfaceProxy* asDeferredSurface() override { return fTextureProxy.get(); }
+    GrTextureProxy* asDeferredTexture() override { return fTextureProxy.get(); }
+    GrRenderTargetProxy* asDeferredRenderTarget() override;
+
 protected:
     GrTextureContext(GrContext*, GrDrawingManager*, sk_sp<GrTextureProxy>, GrAuditTrail*,
                      GrSingleOwner*);