Use the default dispatcher where possible for nested message loops.

Use the default dispatcher where possible for nested message loops.

Notable changes:
 * Add QuitNestedMessageLoop() to client::DispatcherClient, which can be used to
   terminate a nested loop started by RunWithDispatcher().
 * FirstRunDialog is no longer a MessagePumpDispatcher. The default dispatcher
   is used instead, and QuitNestedMessageLoop() is called to terminate the loop
   instead of returning POST_DISPATCH_QUIT_LOOP from the Dispatch() override.

This change was previously committed in r253723 and r254089, but reverted in
r253744 and r254095 because some interactive_ui_tests on windows bots failed on
SimpleMessageBoxViews related change. So this CL aims to reland that CL without
the change in SimpleMessageBoxViews for now.

BUG=342338
R=sky@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=256093

[sadrul, 2014-03-10 21:11:57]

Hi! This is an attempt to reland the changes in
https://codereview.chromium.org/182143002/, without the change in
SimpleMessageBoxViews (which has some lifetime issues, possibly related to the
same reasons that needed it to be refcounted). I will look into fixing that
separately. Mind taking a look at the remaining changes in this CL?

Patchset 1 is the reverted CL, and patchset2 is the CL without the changes in
SimpleMessageBoxViews.

[sky, 2014-03-10 22:43:10]

LGTM

[sadrul, 2014-03-11 01:19:53]

Committed patchset #2 manually as r256093 (presubmit successful).