Add support for GL_EXT_multisampled_render_to_texture.

src/gpu/gl/GrGLInterface.cpp

Index: src/gpu/gl/GrGLInterface.cpp
===================================================================
--- src/gpu/gl/GrGLInterface.cpp	(revision 8935)
+++ src/gpu/gl/GrGLInterface.cpp	(working copy)
@@ -333,7 +333,8 @@
                 return false;
             }
         }
-        if (extensions.has("GL_IMG_multisampled_render_to_texture")) {
+        if (extensions.has("GL_IMG_multisampled_render_to_texture") ||
+            extensions.has("GL_EXT_multisampled_render_to_texture")) {
             if (NULL == fRenderbufferStorageMultisample ||
                 NULL == fFramebufferTexture2DMultisample) {
                 return false;