Make MHTMLGenerationManager observe RenderProcessHosts rather than use notifications.

BUG=170921
TEST=everything still works

Review URL: https://codereview.chromium.org/123993002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242993 0039d316-1c4b-4281-b951-d872f2087c98
Happy New Year!, 2014 edition

R=mal@chromium.org

Review URL: https://codereview.chromium.org/122953003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242994 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242993
SVN changes up to revision 242994