String::NewExternal should not crash the renderer.

The blink may pass a very long string to v8
for making a handle to it. v8 has max string length
limit and creates exception. But NewExternal code does not
check that the handle is null and crashes the renderer.

With the fix the js code receives the exception.

BUG=414615
LOG=N
R=yangguo@chromium.org, yurys@chromium.org

Review URL: https://codereview.chromium.org/603393004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24250 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Script streaming: fix split UTF-8 character handling.

Invalid UTF-8 data can contain too many characters which look like they're part
of a multi-byte character, and that was overflowing a buffer.

BUG=chromium:417891
LOG=n

.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/607043002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24251 ce2b1a6d-e550-0410-aec6-3dcde31c8c00