ANGLE was checking that it had opened the connection  before calling
XSync upon initialization. However, clients may pass in a connection
and ANGLE must sync after creating a GLXWindow with the connection.
This wasn't happening and was causing issues in Chrome.

Bug: chromium:1172803
Change-Id: Ieb2cbfaa226f6d066030f42d8e25b1d9c34779f7
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2757507
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>