Only check element clientWidth on window resize

[JontyUsborne]

checkPortalSize got the clientWidth of the video element, which caused an expensive reflow and potential for dropped frames. Only get the clientWidth when the element resizes.

[TobbeEdgeware]

Does the resize event being triggered properly also for going to full-screen? If so, we may want to use it instead of periodic calls to checkVideoSize in TextTracks.js.

[JontyUsborne]

Yes, the window resize event is triggered when toggling full screen so might be a better option

[LloydW93]

Can you replace all the one-line ifs to be in blocks?
(e.g.)

if (type == 'video') setElementSize();

to

if (type == 'video') {
    setElementSize();
}

Otherwise, lgtm.