Zap the scratch key on non-RT textures when scratch textures are disabled.

Review URL: https://codereview.chromium.org/870103002
diff --git a/src/gpu/GrGpu.cpp b/src/gpu/GrGpu.cpp
index f523632..b7a064c 100644
--- a/src/gpu/GrGpu.cpp
+++ b/src/gpu/GrGpu.cpp
@@ -40,8 +40,8 @@
         return NULL;
     }
 
-    if ((desc.fFlags & kRenderTarget_GrSurfaceFlag) &&
-        !this->caps()->isConfigRenderable(desc.fConfig, desc.fSampleCnt > 0)) {
+    bool isRT = SkToBool(desc.fFlags & kRenderTarget_GrSurfaceFlag);
+    if (isRT && !this->caps()->isConfigRenderable(desc.fConfig, desc.fSampleCnt > 0)) {
         return NULL;
     }
 
@@ -71,6 +71,9 @@
             }
         }
     }
+    if (!this->caps()->reuseScratchTextures() && !isRT) {
+        tex->cacheAccess().removeScratchKey();
+    }
     return tex;
 }