Avoid using std::map::at. This c++11 method call is unavailable on Android. BUG=angle:773 Change-Id: I08064c3f18e387a2ed2496a3fb2912486546843f Reviewed-on: https://chromium-review.googlesource.com/240094 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>