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Each application <> shell connection is represented by a ShellImpl instance
on the shell side. This CL makes the Application watch its ShellPtr for pipe
errors so it can Quit() itself if the shell goes away (shell loop destroyed).

mojo_shell_tests starts using a new method to terminate all shell connections
in this CL, and waits afterward until KeepAlive quits the loop signifying that
all apps are gone. This is done prior to destroying the shell MessageLoop so
that the test ensures apps have all died before the next test. In the future
it would be a test failure if this wasn't a no-op, but we have things in the
shell that don't quit themselves right now.

The shell itself won't wait around for apps to die (as of this CL), but
~MessageLoop will still send the message to Application sides that will quit.

** NOTE ** This does not require an app build target to explicitly add magic
*.cc files to their sources.  It requires selecting an appropriate
mojo_application_{chromium, standalone} library akin to mojo_environment_*.
We can possibly combine these two in the future.

BUG=394477

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=287680

Review URL: https://codereview.chromium.org/394903005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@287894 0039d316-1c4b-4281-b951-d872f2087c98