Promoting the MHTML file extension to primary mappings.

This CL promotes the MHTML file extensions to the MIME primary mappings.
This is to to make MHTML work on Windows.
Windows associates the message/rfc822 MIME type with MHTML files, and
we use the less mail specific multipart/related on the WebKit side.

BUG=83833
TEST=MHTML layout tests should pass on Windows.


Review URL: http://codereview.chromium.org/7077014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@88959 0039d316-1c4b-4281-b951-d872f2087c98