Add locking to all mutable variables of the CL objects in the
front end and pass-through back end to make them thread-safe.
This fixes a crash in a multi-threaded CTS test.

Bug: angleproject:6015
Change-Id: I1d6471c851217639411c434c82acd32d14035291
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2967468
Commit-Queue: John Plate <jplate@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>