Prevent crash in DocumentLoader::dataReceived

This patch prevents a crash in DocumentLoader::dataReceived where the
frame can be destructed in the middle of dataReceived.

BUG=

Review URL: https://chromiumcodereview.appspot.com/23781008

git-svn-id: svn://svn.chromium.org/blink/trunk@157866 bbb929c8-8fbe-4397-9dbb-9b2b20218538