Some compiler versions are unable to resolve
std::string without including <string> header.

Bug: angleproject:4651
Change-Id: Ie2b9d843b55dc46278ed5571d82fa9c413f6b3c9
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2213664
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>