ui: Prefer +/- operators to apply offsets.

ui/gl/gl_surface.h

Index: ui/gl/gl_surface.h
diff --git a/ui/gl/gl_surface.h b/ui/gl/gl_surface.h
index 56d8ae7a126015b3ecba46466c7c87afcc578dc2..0813d7df1a97658a78a8b034b71b0191d0763e85 100644
--- a/ui/gl/gl_surface.h
+++ b/ui/gl/gl_surface.h
@@ -7,6 +7,7 @@
 
 #include <string>
 
+#include "base/callback.h"
 #include "base/memory/ref_counted.h"
 #include "build/build_config.h"
 #include "ui/gfx/native_widget_types.h"
@@ -107,11 +108,17 @@ class GL_EXPORT GLSurface : public base::RefCounted<GLSurface> {
   // Get the GL pixel format of the surface, if available.
   virtual unsigned GetFormat();
 
+  typedef base::Callback<void(const base::TimeTicks timebase,
+                              const base::TimeDelta interval)>
+      UpdateVSyncCallback;
+
   // Get the time of the most recent screen refresh, along with the time
-  // between consecutive refreshes. Returns false when these values are
-  // unavailable.
-  virtual bool GetVSyncParameters(base::TimeTicks* timebase,
-                                  base::TimeDelta* interval);
+  // between consecutive refreshes. The callback is called as soon as
+  // the data is available: it could be immediately from this method,
+  // later via a PostTask to the current MessageLoop, or never (if we have
+  // no data source). We provide the strong guarantee that the callback will
+  // not be called once the instance of this class is destroyed.
+  virtual void GetVSyncParameters(const UpdateVSyncCallback& callback);
 
   // Create a GL surface that renders directly to a view.
   static scoped_refptr<GLSurface> CreateViewGLSurface(
@@ -164,8 +171,7 @@ class GL_EXPORT GLSurfaceAdapter : public GLSurface {
   virtual void* GetDisplay() OVERRIDE;
   virtual void* GetConfig() OVERRIDE;
   virtual unsigned GetFormat() OVERRIDE;
-  virtual bool GetVSyncParameters(base::TimeTicks* timebase,
-                                  base::TimeDelta* interval) OVERRIDE;
+  virtual void GetVSyncParameters(const UpdateVSyncCallback& callback) OVERRIDE;
 
   GLSurface* surface() const { return surface_.get(); }