Update uses of UTF conversions in content/ to use the base:: namespace.

BUG=330556
TEST=no change
TBR=ben@chromium.org

Review URL: https://codereview.chromium.org/121033002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242483 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242483