Refactor TextTrackLoader to properly use the dataReceived callback

We get the data and length in the callback, so just use it directly.

This accidentally fixes a parsing test, by changing the segmentation
of the input received by the WebVTT parser. A separate bug was filed
to fix the problem: http://crbug.com/313222

BUG=312756

Review URL: https://codereview.chromium.org/52533003

git-svn-id: svn://svn.chromium.org/blink/trunk@160965 bbb929c8-8fbe-4397-9dbb-9b2b20218538