Script streaming: UTF-8 handling fix.

The problem was that there can be several multi-byte UTF-8 characters near the
splitting point of the data chunks, and the code didn't handle it properly.

This was also the source of crbug.com/417891 - I thought the crash can only
happen when V8 is passed invalid UTF-8 data, but it can also happen in the
abovementioned case. After the fix, we handle the valid UTF-8 case and also
guard against invalid UTF-8 data.

R=yangguo@chromium.org
BUG=chromium:417891
LOG=N

Review URL: https://codereview.chromium.org/654503002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24547 ce2b1a6d-e550-0410-aec6-3dcde31c8c00