Revert "Remove the pointer from StringImpl"

This reverts commit 83f31bb9cdcb97a23eccf89f5e9b00d894024345.
It seems to break many tests crash on Win7 dbg.

TBR=abarth
BUG=

Review URL: https://codereview.chromium.org/21504002

git-svn-id: svn://svn.chromium.org/blink/trunk@155318 bbb929c8-8fbe-4397-9dbb-9b2b20218538