[ios clean] Move dispatcher out of Browser.

[ios clean] Move dispatcher out of Browser.

This CL moves the dispatcher object out of Browser and makes it a
property of BrowserCoordinatior. The root coordinator in a coordinator
tree must create and own the dispatcher for its tree; child coordinators
get handed a pointer to the dispatcher.

This allows the creation of coordinator trees for generating proxy UI
without risk of dispatcher collisions (by passing a nil dispatcher
into the tree).

Additionally, a 'callableDispatcher' property is added to
BrowserCoordinator. This property is just the coordinator's dispatcher,
cast to |id|. This is a convenience for passing the dispatcher into
view controllers or other objects that don't know about the dispatcher
itself.

BUG=

[marq (ping after 24h), 2017-06-12 09:48:20]

marq@chromium.org changed reviewers:
+ edchin@chromium.org, lpromero@chromium.org, rohitrao@chromium.org

[lpromero, 2017-06-12 13:32:47]

lgtm

[marq (ping after 24h), 2017-06-14 09:26:48]

Ed and Rohit: ping? 

Most of the changes are just updating coordinators to get the dispatcher from
their own property instead of from Browser.

[rohitrao (ping after 24h), 2017-06-14 11:54:08]

It looks like the dispatcher is still owned by Browser, but now we're holding
pointers to it at the BrowserCoordinator level.  Will we eventually remove the
dispatcher from Browser entirely?  Is there a way to prevent individual
coordinators from accidentally using self.browser->dispatcher()?

[marq (ping after 24h), 2017-06-16 13:13:22]

On 2017/06/14 11:54:08, rohitrao (ping after 24h) wrote:
> It looks like the dispatcher is still owned by Browser, but now we're holding
> pointers to it at the BrowserCoordinator level.  Will we eventually remove the
> dispatcher from Browser entirely?  Is there a way to prevent individual
> coordinators from accidentally using self.browser->dispatcher()?

Changed this CL to remove the dispatcher from the browser. What does this break?
Note that the simplest way to do that makes every coordinator hold a strong
reference to the dispatcher.

[lpromero, 2017-06-16 14:47:22]

https://codereview.chromium.org/2935653002/diff/20001/ios/shared/chrome/brows...
File ios/shared/chrome/browser/ui/browser_list/browser.mm (left):

https://codereview.chromium.org/2935653002/diff/20001/ios/shared/chrome/brows...
ios/shared/chrome/browser/ui/browser_list/browser.mm:13: #import
"ios/shared/chrome/browser/ui/commands/command_dispatcher.h"
Nit: you might have a BUILD.gn file to modify too?