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