Have GMs read the GrContext via a setter/getter rather than a global.

gm/gm.h

Index: gm/gm.h
diff --git a/gm/gm.h b/gm/gm.h
index 6cd97b5a2bd75fcd47c0d84bbb6f3c6b86b8f359..2886f8c243fcdb5c24c3c8bf790557848e31ec67 100644
--- a/gm/gm.h
+++ b/gm/gm.h
@@ -16,6 +16,10 @@
 #include "SkString.h"
 #include "SkTRegistry.h"
 
+#if SK_SUPPORT_GPU
+#include "GrContext.h"
+#endif
+
 #define DEF_GM(code) \
     static skiagm::GM*          SK_MACRO_APPEND_LINE(F_)(void*) { code; } \
     static skiagm::GMRegistry   SK_MACRO_APPEND_LINE(R_)(SK_MACRO_APPEND_LINE(F_));
@@ -93,6 +97,10 @@ namespace skiagm {
             fCanvasIsDeferred = isDeferred;
         }
 
+#if SK_SUPPORT_GPU
+        static GrContext* GetGr(/*very nearly const*/ SkCanvas*);
+#endif
+
     protected:
         static SkString gResourcePath;