Don't copy image buffer when calculating size.

src/image/SkImage_Generator.cpp

Index: src/image/SkImage_Generator.cpp
diff --git a/src/image/SkImage_Generator.cpp b/src/image/SkImage_Generator.cpp
index 412f573ba6dbe38d3a9da521fdc48889715db07d..8308b118ed514013625ef9d6664b445bd078b82d 100644
--- a/src/image/SkImage_Generator.cpp
+++ b/src/image/SkImage_Generator.cpp
@@ -32,6 +32,11 @@ public:
     SkImageCacherator* peekCacherator() const override { return fCache; }
     SkData* onRefEncoded(GrContext*) const override;
     sk_sp<SkImage> onMakeSubset(const SkIRect&) const override;
+#if SK_SUPPORT_GPU
+    bool canGenerateTexture(const GrContextThreadSafeProxy& proxy) const override {
+        return fCache->canGenerateTexture(proxy);
+    }
+#endif
     bool getROPixels(SkBitmap*, CachingHint) const override;
     GrTexture* asTextureRef(GrContext*, const GrTextureParams&,
                             SkSourceGammaTreatment) const override;