Rather than storing static DevicePixels dimensions, treat the desired
pixel sizes as CSSPixels and convert them to DevicePixels.
This was originally developed on a mac with a device-to-CSS-pixel ratio
of 2. Running it on another machine with a ratio of 1 made the controls
appear huge.
If the video element has a 'controls' attribute, we now paint some basic
video controls over the video element. If no frame has been decoded yet,
we paint a play button on the center of the element.
If a frame has been decoded, we paint that frame and paint a control bar
on the bottom of the frame. This control bar currently only contains a
play/pause button, depending on the video's playback state. We will only
paint the control bar if the video is paused or hovered.