Allow '\' characters in shader source for WebGL2. TEST=deqp/data/gles3/shaders/preprocessor.html BUG=angleproject:2016 Change-Id: Ia5240a1ff65ebadc15604a5c3eb63042953c43a5 Reviewed-on: https://chromium-review.googlesource.com/540198 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org>