Script streaming: more UTF-8 handing fixes (again).

1) Since we fill the output buffer both from the chunks and the conversion
buffer, it's possible that we run out of space and call CopyCharsHelper with 0
length. The underlying functions don't handle it gracefully, so check there.

2) There was a bug where we used to try to copy too many characters from the
beginning of the data chunk into the conversion buffer. Continuation bytes in
UTF-8 are of the form 0b10XXXXXX. If a byte is bigger than that, it's the first
byte of a new UTF-8 character and we should ignore it.

These two together (or maybe in combination with surrogates) are a probable
reason for crbug.com/420932.

3) The test data was off; \uc481 is \xec\x92\x81.

BUG=420932
LOG=N
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/662003003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24725 ce2b1a6d-e550-0410-aec6-3dcde31c8c00