Add angle_enable_share_context_lock build flag This flags can be used to enable the share context lock. Without this lock, the client need to use gl calls in a threadsafe way. It is true by default. Bug: chromium:1336126 Change-Id: I984f8cfb0379195f6ebe11b0997e401f2421affa Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3780582 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Peng Huang <penghuang@chromium.org>