Convert string16 to base::string16 in content.

BUG=

Review URL: https://codereview.chromium.org/102593002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238447 0039d316-1c4b-4281-b951-d872f2087c98
362 files changed