Add hooks for GL_EXT_debug_marker in gpu

src/gpu/GrGpu.h

Index: src/gpu/GrGpu.h
diff --git a/src/gpu/GrGpu.h b/src/gpu/GrGpu.h
index 00791be054a1e55ceb7c1c36c15c2a465677c7b9..418e84ecdf6941cf09715aa5bb8756a38b368bac 100644
--- a/src/gpu/GrGpu.h
+++ b/src/gpu/GrGpu.h
@@ -458,6 +458,11 @@ private:
     // overridden by backend-specific derived class to perform the resolve
     virtual void onResolveRenderTarget(GrRenderTarget* target) = 0;
 
+    // GrDrawTarget Overrides
+    virtual void onInsertEventMarker(const char* marker) SK_OVERRIDE = 0;
+    virtual void onPushGroupMarker(const char* marker) SK_OVERRIDE = 0;
+    virtual void onPopGroupMarker() SK_OVERRIDE = 0;
+
     // width and height may be larger than rt (if underlying API allows it).
     // Should attach the SB to the RT. Returns false if compatible sb could
     // not be created.