These error strings are stored for recording later we can't depend on them
still being alive so make a copy.

BUG=962439

Change-Id: Ie9156c99b9ce79df4a11087bdcab92d980e959a4
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1610078Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>