Oilpan: move MHTML objects to the heap.

R=haraken@chromium.org,ager@chromium.org,tkent@chromium.org
BUG=340522

Review URL: https://codereview.chromium.org/374053002

git-svn-id: svn://svn.chromium.org/blink/trunk@177719 bbb929c8-8fbe-4397-9dbb-9b2b20218538