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/GrResourceCache2.cpp b/src/gpu/GrResourceCache2.cpp
index d3039733..3918155 100644
--- a/src/gpu/GrResourceCache2.cpp
+++ b/src/gpu/GrResourceCache2.cpp
@@ -218,6 +218,11 @@
     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);