This c++11 standard library class isn't available on Chromium/OSX.
For now we can replace it with std::map.

BUG=angle:773

Change-Id: I0b8ab0de5192a23408755d03df2b9f738f28d762
Reviewed-on: https://chromium-review.googlesource.com/238445Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>