The app "Car Parking Multiplayer" issues a ClientWaitSync() command
without having already flushed the sync object and without an active
context. We should return TIMEOUT immediately rather than attempting to
wait on the sync object, since we can't flush it and it'll never be
signalled.

Bug: angleproject:5613
Bug: angleproject:5656
Test: MultithreadingTest.NoFlushNoContextReturnsTimeout
Change-Id: Ieaf675ca9144f9c851c73b9ca399daaf4ed1cd0f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2693375
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>