DevTools: search in files should't break the search if one file is too big.

v8 fails to convert a 300Mb blob into string handle and throws range exception.
See https://codereview.chromium.org/603393004/

BUG=414615

Review URL: https://codereview.chromium.org/605013004

git-svn-id: svn://svn.chromium.org/blink/trunk@182925 bbb929c8-8fbe-4397-9dbb-9b2b20218538