Bug 833385 - Reformat inheritance lines.

Move delimeters to the next line as per
https://bugzilla.mozilla.org/show_bug.cgi?id=833385#c12

This is better style since it allows new entries to be added/removed
without changing adjacent lines.
Bug 833385 - Move webvtt_node member declaration up.

Fixes a -Wreorder warning on gcc 4.7 and clang 3.2.