ANGLE doesn't recognize the Mesa vendor ID, leading to a crash in
GetVendorString(). This is exposed when trying to capture traces of
ANGLE captures with RenderDoc.

Bug: angleproject:5824
Change-Id: I6c45a01dba78fa6057df26b38ca3db89cacf865e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2809878Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Tim Van Patten <timvp@google.com>