make getContentKey() available in GrGpuResource public interface

include/gpu/GrGpuResource.h

Index: include/gpu/GrGpuResource.h
diff --git a/include/gpu/GrGpuResource.h b/include/gpu/GrGpuResource.h
index bdf6257d4db97242a17c3ef6016663a84683fd92..7896ab3e846a34756a741f90ebbdd7fe1c1e9137 100644
--- a/include/gpu/GrGpuResource.h
+++ b/include/gpu/GrGpuResource.h
@@ -188,6 +188,10 @@ public:
      */
     uint32_t getUniqueID() const { return fUniqueID; }
 
+    /** Returns the current content key for the resource. It will be invalid if the resource has not
+        been cached by its contents. */
+    const GrContentKey& getContentKey() const { return fContentKey; }
+
     /**
      * Attach a custom data object to this resource. The data will remain attached
      * for the lifetime of this resource (until it is abandoned or released).