unordered_map is supposedly faster on deleting items.

BUG=chromium:889545

Change-Id: Id92d9d663e8b9ab2978b8016ef5dccfc93dc104e
Reviewed-on: https://chromium-review.googlesource.com/1255554Reviewed-by: Ali Ijaz Sheikh <ofrobots@google.com>
Commit-Queue: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56354}