[Webapp Refactor] Remove remoting.SessionConnector.

[Webapp Refactor] Remove remoting.SessionConnector.

This is a reland of crrev.com/1047413006

remoting.SessionConnector is currently responsible for creating
the plugin, the signal strategy and the clientSession.  It also
magically disposes the plugin when the clientSession finishes.

However, the session is not exposed to the caller elsewhere, which makes
it hard for the caller to dispose of the session before it is connected,
e.g. Cancel a PIN entry.

It is currently a stateful object that keeps track of the host,
credentials provider, strategy of the created session, which is redundant.

This CL
1. Offloads the creation of the clientSession to the
   remoting.ClientSessionFactory.  It also allow the caller to configure
   a clientSession prior to connecting.  The remoting.ClientSessionFactory
   is essentially stateless except for a few predefined ClientSession
   construction parameters.
2. Allow the caller to have a different instance of
   ClientSession.EventHandler for each instance ClientSession created.
3. Revives remoting.MockClientPlugin and uses it for remoting.ClientSessionFactory
   unittests.

Ownership graph before:
Activity -> SessionConenctor -> ClientSession

Ownership graph after:
Activity -> ClientSession

TBR=jamiewalch
BUG=477522
TEST=All browser tests passed on
https://chromium-swarm.appspot.com/user/tasks?sort=created_ts&state=all&limit=10&task_name=chromoting_integration_tests

Committed: https://crrev.com/2c5de34072226c91d465349cb249d76d16387bef
Cr-Commit-Position: refs/heads/master@{#325887}

[kelvinp, 2015-04-20 17:42:47]

kelvinp@chromium.org changed reviewers:
+ jamiewalch@chromium.org

[kelvinp, 2015-04-20 17:44:35]

Patchset #2 (id:20001) has been deleted

[kelvinp, 2015-04-20 17:47:59]

The CQ bit was checked by kelvinp@chromium.org

[commit-bot: I haz the power, 2015-04-20 17:49:35]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1097133002/40001

[commit-bot: I haz the power, 2015-04-20 18:52:11]

Committed patchset #2 (id:40001)

[commit-bot: I haz the power, 2015-04-20 18:53:00]

Patchset 2 (id:??) landed as
https://crrev.com/2c5de34072226c91d465349cb249d76d16387bef
Cr-Commit-Position: refs/heads/master@{#325887}