It is valid to call eglWaitClient, eglWaitGL and eglWaitNative
when there is no active context in the current thread. Update code
to account for that possibility.

Bug: angleproject:5898
Test: EGLSyncTest.WaitClient*
      EGLSyncTest.WaitGL*
      EGLSyncTest.WaitNative*
Change-Id: I81f05a27f1f641cf8986e2e6c05c183ea8471889
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2849587Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>