plain/text is not a real MIME type.

Fix references to it to be 'text/plain'. Also one instance of 'plain/html'.

BUG=none

Review URL: https://codereview.chromium.org/178153002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253646 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253646