Move EmptyString, kWhitespace and the BOM to base.

This moves EmptyString*, kWhitespace*, and the UTF 8 Byte Order Marker to the base:: namespace.

Many of them just got changed to a default-constructed string when a reference was not required.

I qualified some string16s with base:: when I was changing adjacent code. I need to do another pass to finish these up.

BUG=
TBR=sky@chromium.org

Review URL: https://codereview.chromium.org/89243003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238032 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238032