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This change adds a new top-level menu to the system menu, used to
manipulate and select tabs in the current window. This menu contains
some static items, moved from the Window menu, and then a list of
entries corresponding to open tabs in the current window. Specifically,
this change:

1) Adds a new class, TabMenuBridge, which binds a TabStripModel to a
   NSMenu;
2) Adds a new menu item to the main menu named "Tab";
3) Creates a TabMenuBridge for any tabbed browser window in the
   app controller.

The motivation for this change was twofold: first, to allow easier
access to commands that are currently only present in the tab's
context menu, and second, to allow easier access to tabs in crowded
tabstrips. The first purpose is well-defined and understood but the
second is very much experimental.

Testing:
Build, run, and enjoy the new Tab menu :). There are some unit tests in
TabMenuBridgeTest.* as well.

Future work:
* Renaming the static items to better fit their new context. This was
  not done here to avoid breaking existing user key equivalents, which
  identify items by their full name.
* Exploring other orderings of tabs in this menu.
* Adding other tab actions to this menu, like "Bookmark" or possibly
  page actions.
* Adding a search bar to search within tab titles/origins?
* Adding key equivalents for the static actions (which will resolve
  the actual accessibility bug this commit is for)

Bug: 818261
Change-Id: I790cc2c5a4468719e42b782a318864ecd81bd1f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1728718
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Leonard Grey <lgrey@chromium.org>
Auto-Submit: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683682}