[Downloads/Phone] Add button to icon row for offlining pages
* Generalizes the AppMenu[Adapter/PropertiesDelegate] code to support
up to five items.
* Adds an icon to the phone/compressed tablet menu that is for
offlining a page.
* Hides it behind a flag called: --enable-offline-page-downloading
* Removes an unused icon.
- Not hooked up to anything yet.
- The icon is off-center because the asset provided by go/icons is
off-center. Waiting for a new one from UX.
BUG=
616324
For lint relanding:
TBR=twellington
Committed:
https://crrev.com/35a583985d859d19cd2de1efb7a5b8bb06bcda77
Cr-Commit-Position: refs/heads/master@{#411777}