Devirtualize PageConsole and MHTMLArchive destructors

Devirtualize PageConsole and MHTMLArchive destructors as these classes
are never subclassed nor do they have any virtual function. This avoids
creating unnecessary vtables.

Review URL: https://codereview.chromium.org/41423002

git-svn-id: svn://svn.chromium.org/blink/trunk@160517 bbb929c8-8fbe-4397-9dbb-9b2b20218538