Move Mac-specific String code to WTF

This CL moves several Mac-specific chunks of String code into WTF next to the
cross-platform String code.

R=jamesr
BUG=297477

Review URL: https://codereview.chromium.org/25607002

git-svn-id: svn://svn.chromium.org/blink/trunk@158666 bbb929c8-8fbe-4397-9dbb-9b2b20218538