Chrome makes some ANGLE calls from multiple threads for initializing WebXR
buffers which breaks the assumption that DebugAnnotator11 is only called from a
single thread.  Work around this by only allowing DebugAnnotator11 to log
annotations from the thread used to initialize it.

BUG=995888
BUG=972914

Change-Id: I2241e078031633cafea470f85b7b1ecf1fba8466
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1769057Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>