Merge MIMEHeader.cpp/.h into MHTMLParser

This is only used in one place. Not worth moving to new platform/ directory.

BUG=

Review URL: https://codereview.chromium.org/27044003

git-svn-id: svn://svn.chromium.org/blink/trunk@159527 bbb929c8-8fbe-4397-9dbb-9b2b20218538