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 03dda91e5ea52d1d4b1c017978b68d7cd6a5c09a..61574699fdabd398ef07842000dc72f434cfd6c4 100644
--- a/src/image/SkImage_Generator.cpp
+++ b/src/image/SkImage_Generator.cpp
@@ -30,6 +30,11 @@ 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&,
                             SkDestinationSurfaceColorMode) const override;