Access ChromotingHost::clients_ only on network thread.

Previously ChromotingHost was doing some work on the main thread and
some on the network thread. |clients_| and some other members were
accessed without lock on both of these threads. Moved most of the
ChromotingHost activity to the network thread to avoid possible
race conditions.

BUG=96325
TEST=Chromoting works

Review URL: http://codereview.chromium.org/8495024

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@109556 0039d316-1c4b-4281-b951-d872f2087c98
  virtual void SetUp() {
    encoder_ = new MockEncoder();

    session_ = new MockSession();
    EXPECT_CALL(*session_, SetStateChangeCallback(_));
    connection_ = new MockConnectionToClient(
        session_, &host_stub_, &event_executor_);
    connection_->SetEventHandler(&handler_);

    record_ = new ScreenRecorder(
        &message_loop_, &message_loop_,
        base::MessageLoopProxy::current(),
         &capturer_, encoder_);
   }

  virtual void SetUp() {
    encoder_ = new MockEncoder();

    session_ = new MockSession();
    EXPECT_CALL(*session_, SetStateChangeCallback(_));
    connection_ = new MockConnectionToClient(
        session_, &host_stub_, &event_executor_);
    connection_->SetEventHandler(&handler_);

    record_ = new ScreenRecorder(
        &message_loop_, &message_loop_,
        base::MessageLoopProxy::current(),
         &capturer_, encoder_);
   }
