Add mechanism to proactively purge old resources in GrResourceCache.

This change leaves the feature turned off by default.

Review URL: https://codereview.chromium.org/1032873002
diff --git a/src/gpu/GrContext.cpp b/src/gpu/GrContext.cpp
index 89429f1..48e50b7 100755
--- a/src/gpu/GrContext.cpp
+++ b/src/gpu/GrContext.cpp
@@ -1477,6 +1477,7 @@
     } else {
         fDrawBuffer->flush();
     }
+    fResourceCache->notifyFlushOccurred();
     fFlushToReduceCacheSize = false;
 }