WebGLCompatibility: Fix shader validation using define macros. 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/734218 Reviewed-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>