Assign element IDs to infobar accept and dismiss buttons

This will make it easier to test. One of the methods was still being
used in a unit test so I left it, but I was able to remove the other.

Change-Id: I2f30144dd060a780eae919635447a44b13c3ee2b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5402436


Reviewed-by: Dana Fried <dfried@chromium.org>
Commit-Queue: Alison Gale <agale@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1279997}