Make StringImpl ref counting count by 1 instead of 2

StringImpl incremented its ref count by 2 because the lower bit was used to
track if it was a static string. This meant that we only used half the available
range of ref counts and is not neccesary now that there's bitfields available.

This patch makes it count by 1 and adds a separate field to track static strings.

To avoid bloating the frequently called inline hasOneRef() and deref() I also
moved destruction into a method that only calls delete if the string is not
static.

R=abarth@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23684019

git-svn-id: svn://svn.chromium.org/blink/trunk@157335 bbb929c8-8fbe-4397-9dbb-9b2b20218538