Enter VR immediately upon clicking "Normal mode" or "Spectator mode"

[cvan]

Instead of redirecting to a new page and making the user click an "ENTER VR" button.

[arturitu]

If you click 'Normal' or 'Spectator' before all the site is loaded, appear ENTER VR button, if not you enter directly to VR mode. I think this fix reduce the impact of the #77 issue too (I know this is not the solution for 77 but I think reduce the problem for the end users).

[cvan]

This actually takes now takes the splash page significantly longer to render the text, buttons, etc. (which I filed in #86).

And, per my original comment, the experience should enter immediately upon clicking one of the buttons, not redirected to a screen with another button (ENTER VR).

Here's what should happen:

• If user lands on /index.html, clicking on either "Normal mode" or "Spectator mode" should immediately enter the experience. The interstitial adds unnecessary friction (no matter how slight) and adds more code paths which will affect maintainability.
• If user lands on /index.html?mode=normal, dim the rest of the page and highlight the "Normal mode" button for the user to select (because user-gesture action is required by VRDisplay#requestPresent now).
• If user lands on /index.html?mode=spectator, dim the rest of the page and highlight the "Spectator mode" button for the user to select.