Add sidebar views implementation using ExtensionSystem SidebarManager

Add sidebar views implementation using ExtensionSystem SidebarManager

Allow browser extension actions to show their popup inside a
SidebarContainer.

BUG=477424

[ltilve, 2015-06-09 09:27:25]

ltilve@igalia.com changed reviewers:
+ kalman@chromium.org, rdevlin.cronin@chromium.org

[ltilve, 2015-06-09 09:52:47]

Add sidebar views to support extensions showing their popup contents into a
sidebar.

Depends on the implementation at https://codereview.chromium.org/1168383002/

[benjamin.j.mccann, 2015-07-10 19:45:56]

Thanks for the reviews on https://codereview.chromium.org/1168383002/! I was
wondering if it might make sense to do a round of review on this one as well
while that's pending so that we can keep busy addressing anything that's known
while the ball's out of our court on 1168383002

[Devlin, 2015-07-10 20:20:58]

On 2015/07/10 19:45:56, benjamin.j.mccann wrote:
> Thanks for the reviews on https://codereview.chromium.org/1168383002/! I was
> wondering if it might make sense to do a round of review on this one as well
> while that's pending so that we can keep busy addressing anything that's known
> while the ball's out of our court on 1168383002

My concern is mostly that these CLs depend pretty heavily on the first one, so
any comments made on this CL would be potentially moot with the changes in the
other.  I'd prefer to get the first one stable, and then work on these.  (That
said, the views and cocoa implementations can probably be reviewed in parallel,
since they have very little overlap.)