New tab context menu option adds tab to group.

Add group parameter to InsertWebContentsAt so new tabs can be added to a
group upon creation. All callers (except for the chain that allows the
new tab context menu option to function as intended) default to a null
group for now; updating callers with desired behavior will happen in
future CLs.

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: I9affcfb000fa576888354158e76ba6db13cc0397
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1431917
Commit-Queue: Taylor Bergquist <tbergquist@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644127}
12 files changed