This avoids potential issues with returning local variables. It
also solves potential threading conflicts by returning the string
value instead of a pointer. The code should be optimized with RVO
with a modern c++ compiler.

Bug: angleproject:2601
Change-Id: I8a01702d2675a17dd060f27920105efab0c49454
Reviewed-on: https://chromium-review.googlesource.com/c/1415910Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>