Add base namespace to some instances of string16.

Adds the base:: qualifier to more instances of string16 in chrome/browser.

BUG=

Review URL: https://codereview.chromium.org/104043005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239257 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239257