Add GL_CHROMIUM_framebuffer_multisample support.

Review URL: http://codereview.appspot.com/4287072/



git-svn-id: http://skia.googlecode.com/svn/trunk@984 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gpu/src/GrGpuGL.h b/gpu/src/GrGpuGL.h
index 0134407..bd6250b 100644
--- a/gpu/src/GrGpuGL.h
+++ b/gpu/src/GrGpuGL.h
@@ -165,9 +165,10 @@
 
     GrGLuint fAASamples[4];
     enum {
-        kNone_MSFBO = 0,
-        kDesktop_MSFBO,
-        kApple_MSFBO
+        kNone_MSFBO = 0,  //<! no support for MSAA FBOs
+        kDesktopARB_MSFBO,//<! GL3.0-style MSAA FBO (GL_ARB_framebuffer_object)
+        kDesktopEXT_MSFBO,//<! earlier GL_EXT_framebuffer* extensions
+        kAppleES_MSFBO,   //<! GL_APPLE_framebuffer_multisample ES extension
     } fMSFBOType;
 
     // Do we have stencil wrap ops.