Backslash as line-continutation character is not permitted to be used
within a pre-processor directive for WebGL1.

BUG=angleproject:2093

Change-Id: I649a914b9bc544a3e4f9e7eff23b95a62c9b7008
Reviewed-on: https://chromium-review.googlesource.com/734218Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Bryan Bernhart <bryan.bernhart@intel.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>