Make a single GrSingleOwner in GrContext

include/gpu/GrContext.h

Index: include/gpu/GrContext.h
diff --git a/include/gpu/GrContext.h b/include/gpu/GrContext.h
index 39477e0db81fbdc0875aec82f4220664336be11f..c9b7264e09f9bdcab14c16dd31516d1f44aa6502 100644
--- a/include/gpu/GrContext.h
+++ b/include/gpu/GrContext.h
@@ -392,7 +392,7 @@ private:
     SkMutex                         fTestPMConversionsMutex;
 
     // In debug builds we guard against improper thread handling

[robertphillips, 2016/01/07 16:44:11]

// This guard is passed to the GrDrawingManager and, from there, to all the
GrDrawContexts. 

?

-    SkDEBUGCODE(mutable GrSingleOwner fSingleOwner;)
+    mutable GrSingleOwner fSingleOwner;
 
     struct CleanUpData {
         PFCleanUpFunc fFunc;