*re-land with build fix for Clang*

These helper functions mirror some utilities in Chrome, for splitting
strings along whitespace, or reading file contents into a string.

Also remove the hack for skipping the doubly-defined GLX header.

BUG=angleproject:892,angleproject:998

Change-Id: Ife43fbf5035a3be7820460bea1b26d0e632a4fb0
Reviewed-on: https://chromium-review.googlesource.com/272518Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>