Cleanup: Move kChromeUIScheme constant into content namespace.

BUG=None
TEST=None, no functional changes
TBR=jam@chromium.org

Review URL: https://codereview.chromium.org/149643010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250244 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250244