Add R_8 GPU memory buffers format.

gpu/GLES2/extensions/CHROMIUM/CHROMIUM_image.txt

 Name
 
     CHROMIUM_image
 
 Name Strings
 
     GL_CHROMIUM_image
 
 Version
 
     Last Modifed Date: Oct 7, 2014
 
 Dependencies
 
     OpenGL ES 2.0 is required.
 
     EXT_texture_format_BGRA8888 affects the definition of this extension.
+    ARB_texture_rg affects the definition of this extension.
 
 Overview
 
     This extension defines a new resource type that is suitable for
     sharing 2D arrays of image data between client APIs.
 
 Issues
 
     None
 
@@ skipping 10 matching lines @@
 
     Create an image from <buffer> with width equal to <width> and
     height equal to <height> and format equal to <internalformat>.
 
     Returns a unique identifier for the image that could be used in
     subsequent operations.
 
     INVALID_VALUE is generated if <width> or <height> is nonpositive.
 
     INVALID_ENUM is generated if <internalformat> is not one of
-    RGB, RGBA or GL_BGRA_EXT.
+    R8, RGB, RGBA or BGRA_EXT.
 
     void DestroyImageCHROMIUM(GLuint image_id)
 
     Frees the image previously created by a call to CreateImageCHROMIUM.
 
     INVALID_OPERATION is generated if <image_id> is not a valid image id.
 
 Dependencies on EXT_texture_format_BGRA8888
 
     If EXT_texture_format_BGRA8888 is not supported:
-     * delete any reference to the GL_BGRA_EXT format.
+     * delete any reference to the BGRA_EXT format.
+
+Dependencies on ARB_texture_rg
+
+    If ARB_texture_rg is not supported:
+     * delete any reference to the R8 format.
 
 Errors
 
     None.
 
 New State
 
     None.
 
 Revision History
 
     5/9/2013    Documented the extension
     4/30/2014   Moved usage flag to creation function.
     10/7/2014   Remove map/unmap API.
-    4/6/2015    Add GL_BGRA_EXT format.
+    4/6/2015    Add BGRA_EXT format.
+    2/7/2015    Add R8 format.