Return a size of 0 for external format GrSurfaces.

This fixes a case where Chrome was hitting the assert in ComputeSize that
colorSize > 0. In general this shouldn't affect our memory/budget tracking
since external textures are generally given to us as wrapped borrowed textures
so they're memory was not counted against our budget to begin with.

Change-Id: I2fd4eaaab52d9c3dc1af3a8f5b59a1b1f3c5dbb7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/256656
Reviewed-by: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>

include/gpu/GrContext.h

@@ -365,7 +365,8 @@ public:
     void storeVkPipelineCacheData();
 
     // Returns the gpu memory size of the the texture that backs the passed in SkImage. Returns 0 if
-    // the SkImage is not texture backed.
+    // the SkImage is not texture backed. For external format textures this will also return 0 as we
+    // cannot determine the correct size.
     static size_t ComputeImageSize(sk_sp<SkImage> image, GrMipMapped, bool useNextPow2 = false);
 
     /*

src/gpu/GrSurface.cpp

@@ -21,6 +21,12 @@ size_t GrSurface::ComputeSize(const GrCaps& caps,
                               int colorSamplesPerPixel,
                               GrMipMapped mipMapped,
                               bool binSize) {
+    // For external formats we do not actually know the real size of the resource so we just return
+    // 0 here to indicate this.
+    if (format.textureType() == GrTextureType::kExternal) {
+        return 0;
+    }
+
     size_t colorSize;
 
     if (binSize) {