Want to test several scenarios:
* Go to a WebXR page with GVR uninstalled.  Expect supportsSession(immersive)==true, and UI shows when we requestSession(immersive).  No UI if we requestSession(non-immersive), no UI if we requestSession(ar).
* Go to WebVR page with GVR uninstalled.  Expect that UI shows, and we get orientation device info.  Should device claim to support presentation?
* Go to WebXR page with GVR installed.  Expect no UI.  Expect supportsSession(immersive)==true.  Expect that we don't start GVR until requestSession(immersive).
* Go to WebVR page with GVR installed.  Expect we get display info initially corresponding to GVR device.

I believe we have some similar tests already for VR browsing.  https://cs.chromium.org/chromium/src/chrome/android/javatests/src/org/chromium/chrome/browser/vr/VrInstallUpdateInfoBarTest.java?q=vrinstallupdate&sq=package:chromium&dr=CSs&l=119

Finally, if we install GVR through UI, expect Chrome to restart, then we are in the GVR installed state when we get back.

It would be nice to have tests that cover the DFM and ARCore install flows similar to this, since it should be similar patterns/test cases.  I'm not sure what exists currently for these.