Adding a WebGL panel that hosts the graphics replay stuff.
Only appears when WebGL events are present.
It'd be nice to have a special marker in files indicating whether it is
replayable and only show when it actually is.

Some cleanup of the DOM was required in the graphics replay code to get it
resizing correctly inside of the panel. Nothing major. There was also an
unused file checked in that has been removed.

Also fixes #367.
Merge pull request #402 from google/webglpanel

Adding a WebGL panel that hosts the graphics replay stuff.