Correct data size argument type in resource loading path to unsigned

int is used for ThreadableLoaderClient::didReceiveData()'s dataLength
argument and some of its implementations are calling strlen when
dataLength is -1. It never happens. We should give appropriate type to
the data path.

Chromium side CL: https://codereview.chromium.org/615823002/

R=tkent,kouhei
BUG=418885

Review URL: https://codereview.chromium.org/618583002

git-svn-id: svn://svn.chromium.org/blink/trunk@183121 bbb929c8-8fbe-4397-9dbb-9b2b20218538