GL_MULTISAMPLE is not a real state in GLES but existed for the D3D
renderers to sync the rasterizer state more easily.  Instead of syncing it
in StateManagerGL, rely on the default value of GL_TRUE in Desktop GL.

All tests in dEQP-GLES3.functional.multisample.* now pass.

BUG=angleproject:885
BUG=angleproject:883

Change-Id: Idbe5fbc1b43a6e66008fe87da41601277dbe7311
Reviewed-on: https://chromium-review.googlesource.com/298441Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>