Parser & internalization fix: ensure no heap allocs during GetString(Handle<String>).

The bug has always been there: when the parser is operating in the "immediately
internalize" mode and calls GetString, we get FlatContent of a string and then
do heap allocation.

The bug was uncovered by https://codereview.chromium.org/693803004/ (which put
the parser to the "immediately internalize" mode more often), but looking at the
code, it's possible that it can happen in other cases too.

This CL makes AstValueFactory handle this situation gracefully: it won't try to
internalize inside GetString(Handle<String>); it's unnecessary anyway since we
have the Handle<String> already.

R=rossberg@chromium.org
BUG=

Review URL: https://codereview.chromium.org/699343004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25155 ce2b1a6d-e550-0410-aec6-3dcde31c8c00