Use the default dispatcher where possible for nested message loops.
Use the default dispatcher where possible for nested message loops.

This is a re-land of crrev.com/256666.

* SimpleMessageBoxViews is no longer 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.

* Always use dialog widget window's dispatcher-client.
There is no compelling reason for choosing the parent's dispatcher-
client over the dialog's dispatcher-client. Furthermore choosing
the wrong dispatcher-client causes a CHECK failure in WinDbg
interactive_ui_tests:CloseWithModalDialog.

R=sadrul@chromium.org
TBR=msw@chromium.org, sadrul@chromium.org, sky@chromium.org

BUG=351393
TEST=trybot and manual testing on Chrome OS.

Review URL: https://codereview.chromium.org/195763014

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@257032 0039d316-1c4b-4281-b951-d872f2087c98

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