Enable anti-alias in WebGL if only GL_EXT_multisampled_render_to_texture is supported.

BUG=429559

Review URL: https://codereview.chromium.org/671643005

git-svn-id: svn://svn.chromium.org/blink/trunk@184901 bbb929c8-8fbe-4397-9dbb-9b2b20218538