Use scratch keys for stencil buffers.

BUG=skia:2889

Committed: https://skia.googlesource.com/skia/+/91175f19664a62851da4ca4e0984a7c7c45b258f

Review URL: https://codereview.chromium.org/747043004
diff --git a/src/gpu/GrGpuResourceCacheAccess.h b/src/gpu/GrGpuResourceCacheAccess.h
index c55bb07..e220e5f 100644
--- a/src/gpu/GrGpuResourceCacheAccess.h
+++ b/src/gpu/GrGpuResourceCacheAccess.h
@@ -50,6 +50,12 @@
     const GrResourceKey& getScratchKey() const { return fResource->fScratchKey; }
 
     /**
+     * If the resource has a scratch key, the key will be removed. Since scratch keys are installed
+     * at resource creation time, this means the resource will never again be used as scratch.
+     */
+    void removeScratchKey() const { fResource->removeScratchKey();  }
+
+    /**
      * If the resource is currently cached by a content key, the key is returned, otherwise NULL.
      */
     const GrResourceKey* getContentKey() const {