unordered_map was showing up as a hotspot in replay profiling.

Bug: angleproject:4916
Change-Id: I840371313ab51faa088664b9840c7ef3f998fcad
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2334094Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Manh Nguyen <nguyenmh@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>