Make GrFakeRefObj::fNext atomic.

This lets you make them on different threads simultaneously.

Change-Id: Ife4228ae4721b35c718ece1a30cc5686f3690c13
Reviewed-on: https://skia-review.googlesource.com/48800
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

tools/gpu/gl/debug/GrFakeRefObj.h

@@ -8,6 +8,7 @@
 #ifndef GrFakeRefObj_DEFINED
 #define GrFakeRefObj_DEFINED
 
+#include <atomic>
 #include "SkTypes.h"
 #include "gl/GrGLInterface.h"
 
@@ -26,7 +27,7 @@ public:
         , fDeleted(false) {
 
         // source for globally unique IDs - 0 is reserved!
-        static int fNextID = 0;
+        static std::atomic<int> fNextID{0};
 
         fID = ++fNextID;
     }