Make GrGpuResource::gpuMemorySize non-virtual w/ onGpuMemorySize virtual impl

BUG=skia:2889

Review URL: https://codereview.chromium.org/702413003
diff --git a/src/gpu/GrResourceCache.cpp b/src/gpu/GrResourceCache.cpp
index 7083cf3..a73d117 100644
--- a/src/gpu/GrResourceCache.cpp
+++ b/src/gpu/GrResourceCache.cpp
@@ -16,6 +16,7 @@
 ///////////////////////////////////////////////////////////////////////////////
 
 void GrGpuResource::didChangeGpuMemorySize() const {
+    fGpuMemorySize = kInvalidGpuMemorySize;
     if (this->cacheAccess().isInCache()) {
         fCacheEntry->didChangeResourceSize();
     }