Parser & internalizing: efficiency fixes.

1) In ParseLazy, we're already using the heap, so we can also tell
AstValueFactory to internalize immediately. This is more efficient.

2) No need to collect the values in values_ and strings_ if they're already
internalized.

3) No need to collect AstValues which are strings in values_ since they don't
need to be internalized (the underlying strings will be internalized
separately).

BUG=429168
LOG=N
R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/693803004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25109 ce2b1a6d-e550-0410-aec6-3dcde31c8c00