Restricts MHTML loading to top frames.
Also disabling JavaScript for MHTML documents.
Because JavaScipt is now disabled, MHTML layout tests had to be changed
not to use JavaScript anymore.

BUG=330663
TEST=Run MHTML layout tests.
R=abarth@chromium.org, jschuh@chromium.org

Review URL: https://codereview.chromium.org/161383002

git-svn-id: svn://svn.chromium.org/blink/trunk@169499 bbb929c8-8fbe-4397-9dbb-9b2b20218538