The std::map became a significant bottleneck for draw calls.

BUG=angleproject:959

Change-Id: I28176d1474fe5cff05018f89878ea8b2a3e3f9f1
Reviewed-on: https://chromium-review.googlesource.com/288702Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>