Drop "Main document" support

Spec: http://www.w3.org/2012/sysapps/app-lifecycle/#a-single-entry-point-to-the-application
SSIA. This patch removes the related functionality and tests.
Application class required quite deep modification, now it is inheriting
'content::RenderProcessHostObserver' to watch RPH lifecycle.
Merge pull request #1961 from pozdnyakov/Remove_MainDocument

Drop "Main document" support