Add context menu submenu to add tab to existing tab group.

Reuses (and slightly simplifies) some of the logic used to add tabs to
new tab groups.

This CL is part of the prototype of the above-described tab groups
feature.  The prototype will allow users to create and manipulate groups
primarily via tab context menus, and will display tab group affiliation
in the tabstrip.  Future work will include persisting and syncing groups,
manipulating groups via tab dragging, and a dropdown menu for the group
headers. See go/chrome-tab-groups-design

Bug: 905491

Change-Id: Id36b896ab5e1cfb51f85b6e1f4fb931670fa2ff1
Commit-Queue: Taylor Bergquist <>
Reviewed-by: Erik Chen <>
Reviewed-by: Jesse Doherty <>
Reviewed-by: Bret Sepulveda <>
Cr-Commit-Position: refs/heads/master@{#623533}
12 files changed