Use a TextTrackLoaderClient reference (not pointer)

It's never null, so remove all doubt for future readers of the code.

BUG=313287

Review URL: https://codereview.chromium.org/48113029

git-svn-id: svn://svn.chromium.org/blink/trunk@160970 bbb929c8-8fbe-4397-9dbb-9b2b20218538