Some cleanups for the message_loop.h/cc.

Some cleanups for the message_loop.h/cc.

The cleanups include:
 * Remove some #includes and typedefs from message_loop.h
 * Remove MessageLoop::Observer, and add MessageLoopForUI:Observer instead.
 * Wrap all of MessageLoopForUI inside !defined(OS_NACL).
 * Remove some pump_*() accessors (e.g. pump_win(), pump_libevent()), and
   use static_cast<> instead where necessary.
 * Add a MessagePumpForIO typedef in message_loop.cc for the various IO
   message-pumps, and remove MESSAGE_PUMP_IO macro for creating the io pumps.
 * Simplify the #ifdef's for MESSAGE_PUMP_UI macro.

BUG=none
R=darin@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=265451

[sadrul, 2014-04-22 20:26:30]

Do these look like reasonable cleanups? There are still quite a bit of #ifdef's
after this change, but I think the code is a bit more readable with this. WDYT?
(I considered splitting up message_loop.cc|h into multiple files, but am not
sure if that'd be worth it, since it will result in a few very small files).

[darin (slow to review), 2014-04-22 22:49:48]

LGTM

[sadrul, 2014-04-23 00:55:27]

Committed patchset #9 manually as r265451 (presubmit successful).