Test Keeping NaCl plugins used in app background pages alive when active.

Activity in Native Client plugins results in IPC messages 
sent to the BrowserPpapiHostImpl and routed to call 
extensions::ProcessManager::KeepaliveImpulse. 

Testing patch, builds on implementation. See: 
https://codereview.chromium.org/61063003/ Implementation.
https://codereview.chromium.org/105873003/ Cumulative patch. 

Design doc: 
https://docs.google.com/a/chromium.org/document/d/1mI0lS1rfAf-BAGLmWAEcWy37Xq9dOvgfMx8OqeUMXts/edit# 

BUG=298339

Review URL: https://codereview.chromium.org/111563006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242061 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242061