Clean up StringImpl bits

After refactoring StringImpl, we're now only using 4 bits of flags. This CL
cleans up the StringImpl flags to make that clearer.

R=eseidel

Review URL: https://chromiumcodereview.appspot.com/19475006

git-svn-id: svn://svn.chromium.org/blink/trunk@154793 bbb929c8-8fbe-4397-9dbb-9b2b20218538