A previous change based on ESSL 1.00 spec had made it an error in all cases when an extension directive appeared in a shader after the first non-preprocessor token. However, this is incorrect for WebGL 1.0 so adding warning case for WebGL. BUG=chromium:971660 Change-Id: I026fe60e8b1876de65b001b676f7a0552739a20c Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1648661Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Kai Ninomiya <kainino@chromium.org> Commit-Queue: Tobin Ehlis <tobine@google.com> (cherry picked from commit 35b25fc6) Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1651081
