Move op memory storage to GrContext (take 2)

TBR=bsalomon@google.com

Change-Id: I4a448694d4114d83cd3a720cfc8bd37de51733d1
Reviewed-on: https://skia-review.googlesource.com/135707
Reviewed-by: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
diff --git a/src/gpu/GrMemoryPool.h b/src/gpu/GrMemoryPool.h
index 67991d3..15399b6 100644
--- a/src/gpu/GrMemoryPool.h
+++ b/src/gpu/GrMemoryPool.h
@@ -146,6 +146,8 @@
 
     void release(std::unique_ptr<GrOp> op);
 
+    bool isEmpty() const { return fMemoryPool.isEmpty(); }
+
 private:
     GrMemoryPool fMemoryPool;
 };