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