Add method to check if all Activities were destroyed

Needed to know when to properly destroy classes.

BUG=329287

Review URL: https://codereview.chromium.org/111343007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241503 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241503