Switch MHTMLGenerationManager to use a Callback.

Relanding:
MHTMLGenerationManager now uses a callback instead of a notification.

BUG=None
TEST=MHTML generation (via extension API should still work).
TBR=jam

Review URL: http://codereview.chromium.org/8674002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@111328 0039d316-1c4b-4281-b951-d872f2087c98
6 files changed