Use scratch keys for stencil buffers.

src/gpu/GrResourceCache2.cpp

Index: src/gpu/GrResourceCache2.cpp
diff --git a/src/gpu/GrResourceCache2.cpp b/src/gpu/GrResourceCache2.cpp
index d30397337fcd96a0abaa7a3d3f4587857bfdc3cc..39181554d23705bc863e5f0ad4c3d0b1dc88dd77 100644
--- a/src/gpu/GrResourceCache2.cpp
+++ b/src/gpu/GrResourceCache2.cpp
@@ -218,6 +218,11 @@ GrGpuResource* GrResourceCache2::findAndRefScratchResource(const GrResourceKey&
     return resource;
 }
 
+void GrResourceCache2::willRemoveScratchKey(const GrGpuResource* resource) {
+    SkASSERT(resource->cacheAccess().isScratch());
+    fScratchMap.remove(resource->cacheAccess().getScratchKey(), resource);
+}
+
 bool GrResourceCache2::didSetContentKey(GrGpuResource* resource) {
     SkASSERT(!fPurging);
     SkASSERT(resource);