Making index implicit by using vector instead of map.

A redo of cl/1458676 fixing the tests.

Bug: 929155
Change-Id: I78afa11d8aa8444d64d087a85744d61228015dc4
Reviewed-on: https://chromium-review.googlesource.com/c/1471238


Commit-Queue: Alexandre Frechette <frechette@chromium.org>
Reviewed-by: anthonyvd <anthonyvd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633424}