Renamed to MultipleDrawBuffers and added shader loading from file.

BUG=angle:521

Change-Id: Iffeb589a95eee4409d3624c56fe3a5e4a7d5be63
Reviewed-on: https://chromium-review.googlesource.com/188724Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>