GitLab

Do a couple small cleanups in ExtensionsMenuView:
- Update includes and forward declarations (removing unused, adding
  missing)
- Add function comments
- Remove unused function CreateFixedSizeIconView() (the last usage was
  removed in crrev.com/7127b171)

Bug: None
Change-Id: I83513678011b6adf825377def47003a677e5fb17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1879605
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Peter Boström <pbos@chromium.org>
Reviewed-by: Charlene Yan <cyan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709315}