Apply toFoo generation macro to TrackBase classes

As a step to use toFoo, this cl introduces DEFINE_TRACK_TYPE_CASTS, and use it.

BUG=309516

Review URL: https://codereview.chromium.org/392193002

git-svn-id: svn://svn.chromium.org/blink/trunk@178265 bbb929c8-8fbe-4397-9dbb-9b2b20218538