Vulkan: Submit commands when Sync objects are initialized This CL calls ContextVk::flushImpl() after the sync object (VkEvent) has been created and initialized, so it has a valid Serial when it's waited on later. This change allows us to remove any tracking of pending sync objects, since all sync objects will be flushed and submitted by the end of SyncHelper::initialize(). Bug: angleproject:6464 Test: MultithreadingTest.CreateFenceThreadAClientWaitSyncThreadBDelayedFlush Change-Id: I47e7ced452727b434ed974368311fb3439a107c9 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3200274 Reviewed-by: Charlie Lao <cclao@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Tim Van Patten <timvp@google.com>