gpu: Add GpuFence framework.

ui/gl/gl_fence.h

Index: ui/gl/gl_fence.h
diff --git a/ui/gl/gl_fence.h b/ui/gl/gl_fence.h
index c3788eb8535928d8938237653f47eaec565dc8c0..31c16f72ab2170ba82a41a33ba9fc31d50168c1e 100644
--- a/ui/gl/gl_fence.h
+++ b/ui/gl/gl_fence.h
@@ -34,6 +34,12 @@ class GL_EXPORT GLFence {
   // Loses the reference to the fence. Useful if the context is lost.
   virtual void Invalidate();
 
+  // Returns true if signaling is supported.
+  virtual bool SignalSupported();
+
+  // Signal the fence.
+  virtual void Signal();
+
  private:
   DISALLOW_COPY_AND_ASSIGN(GLFence);
 };