Don't copy image buffer when calculating size.

include/gpu/GrContext.h

Index: include/gpu/GrContext.h
diff --git a/include/gpu/GrContext.h b/include/gpu/GrContext.h
index 67f6c93620029afbaf7822554457c7bd2caab039..9021346bd939f08bea529e1122c95edc566bc740 100644
--- a/include/gpu/GrContext.h
+++ b/include/gpu/GrContext.h
@@ -486,6 +486,9 @@ private:
  * proxy does not access the 3D API (e.g. OpenGL) that backs the generating GrContext.
  */
 class GrContextThreadSafeProxy : public SkRefCnt {
+public:
+    uint32_t uniqueID() const { return fContextUniqueID; }
+
 private:
     GrContextThreadSafeProxy(sk_sp<const GrCaps> caps, uint32_t uniqueID)
         : fCaps(std::move(caps))