Add 8bit Content-Transfer-Encoding support for reading MHTML files

R=abarth@chromium.org
BUG=274232

Review URL: https://codereview.chromium.org/22292008

git-svn-id: svn://svn.chromium.org/blink/trunk@156720 bbb929c8-8fbe-4397-9dbb-9b2b20218538