alert() needs to block on a single boolean result and is currently using
a nested loop to process async messages while waiting for a reply from
browser process. The nested loop adds a huge complexity to the subsystem as
it requires support for reentrancy and reordering in handling every message.

Especially, when ImeThread feature was enabled, it was causing a DCHECK
failure and a hang because some messages got unexpectedly reordered.

Instead of landing a workaround for ImeThread case, this removes nested
message loop use case for RenderFrameImpl.

Other changes here includes:
1) Fix ExtensionApiTest not to send async messages and wait for result
   while alert dialog is showing up. This would hang otherwise.
2) Change UnloadController / FastUnloadController to check whether
   beforeunload events have already been fired in the case of devtools.
   We were firing beforeunload events twice when closing devtools
   incorrectly even before this change, but closing process continued
   because async messages were allowed. This fixes
   DevToolsBeforeUnloadTest::TestUndockedDevToolsApplicationClose.
3) For 2), change Browser and BrowserTabStripModel to call non-static
   member functions.
4) Fix UnloadTest by removing the two second wait limit. Sometimes it took
   more than 2 seconds just to close the browser, but the test could close
   the browser because async messages were allowed. Now the test fails if
   before unload dialog shows up, so we prevent it by running an infinite
   loop.

BUG=604675

Review-Url: https://codereview.chromium.org/1931793002
Cr-Commit-Position: refs/heads/master@{#394883}