Various XR fixes

[Manishearth]

Mostly making sure we expose the correct things on the XRSession interface.

No backend currently supports visibility state changes, and it's tricky to do this for Hololens because currently we must pump the event loop to notice these events, and only rAF pumps the event loop right now, which means that applications which choose to stop running rAF when blurred will have a problem.

r? @jdm

[highfive]

Heads up! This PR modifies the following files:

• @asajeffrey: components/script/dom/webidls/XRSession.webidl, components/script/dom/xrsession.rs
• @KiChjang: components/script/dom/webidls/XRSession.webidl, components/script/dom/xrsession.rs

[jdm]

@bors-servo r+

[bors-servo]

📌 Commit 49a8ed0 has been approved by jdm

[bors-servo]

⌛ Testing commit 49a8ed0 with merge 9a8815e...

[bors-servo]

💔 Test failed - status-taskcluster

[jdm]

@bors-servo retry

[bors-servo]

💣 Failed to start rebuilding: Unknown error

[bors-servo]

⌛ Testing commit 49a8ed0 with merge a84d4ab...

[bors-servo]

💔 Test failed - linux-rel-css

[CYBAI]

@bors-servo retry

• #22955

[bors-servo]

⚡ Previous build results for status-taskcluster are reusable. Rebuilding only linux-rel-css, linux-rel-wpt...

[bors-servo]

☀️ Test successful - linux-rel-css, linux-rel-wpt, status-taskcluster
Approved by: jdm
Pushing a84d4ab to master...

[CYBAI]

Hmm, that's weird. After this PR merged, I pull the latest master.

I got following build error:

   Compiling script_layout_interface v0.0.1 (/Users/cybai/codespace/mozilla/servo/components/script_layout_interface)
   Compiling layout_traits v0.0.1 (/Users/cybai/codespace/mozilla/servo/components/layout_traits)
   Compiling layout_2013 v0.0.1 (/Users/cybai/codespace/mozilla/servo/components/layout)
   Compiling constellation v0.0.1 (/Users/cybai/codespace/mozilla/servo/components/constellation)
    Building [=====================================================> ] 601/611: mozjs_sys(build), osmesa-src(buil
error[E0432]: unresolved import `webxr_api::EnvironmentBlendMode`
  --> components/script/dom/xrsession.rs:53:11
   |
53 |     self, EnvironmentBlendMode, Event as XREvent, Frame, SelectEvent, Session, Visibility,
   |           ^^^^^^^^^^^^^^^^^^^^ no `EnvironmentBlendMode` in the root

error[E0599]: no method named `environment_blend_mode` found for type `webxr_api::session::Session` in the current scope
  --> components/script/dom/xrsession.rs:91:33
   |
91 |             blend_mode: session.environment_blend_mode().into(),
   |                                 ^^^^^^^^^^^^^^^^^^^^^^ method not found in `webxr_api::session::Session`

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0432, E0599.
For more information about an error, try `rustc --explain E0432`.
error: could not compile `script`.

To learn more, run the command again with --verbose.
Build FAILED in 0:23:28

Then, I opened Cargo.lock and observed the commit hash id of webxr-api is changed back to the old one. The most interesting thing was, I didn't see the hash change on terminal but on VSCode. 🤔

After checkout the hash id to the latest one, I can build Servo successfully 🤔