gpu: Add GpuFence framework.

gpu/GLES2/extensions/CHROMIUM/CHROMIUM_fence.txt

+Name
+
+    CHROMIUM_fence
+
+Name Strings
+
+    GL_CHROMIUM_fence
+
+Version
+
+    Last Modified Date: September 30, 2016
+
+Dependencies
+
+    OpenGL ES 2.0 is required.
+
+Overview
+
+    This extension introduces the concept of "fence objects".
+
+Issues
+
+    None
+
+New Tokens
+
+    None
+
+New Procedures and Functions
+
+    The command
+
+        GLuint CreateFenceCHROMIUM(ClientFence fence)
+
+    creates a fence from <fence> and returns a unique identifier for the
+    fence that could be used in subsequent operations.
+
+    To destroy a fence, call
+
+        void DestroyFenceCHROMIUM(GLuint fence_id)
+
+    INVALID_OPERATION is generated if <fence_id> is not a valid fence id.
+
+Errors
+
+    None.
+
+New State
+
+    None.
+
+Revision History
+
+    9/30/2016    Documented the extension