Bug 1767449 - Turn on screen.orientation.lock in Nightly. r=smaug

I would like to enable this API in Nightly for testing.

Supported in GeckoView and Windows tablet mode. Other platforms are no
affect by this preference. WPT [*1] runs on non-supported platform, so
a lot of tests are failure. (GeckoView passes a lot of tests [*2].)

Also, there is an issue of hidden document [*3], it isn't fixed by spec
side yet.

*1 https://wpt.fyi/results/screen-orientation
*2 https://searchfox.org/mozilla-central/source/testing/web-platform/meta/screen-orientation
*3 w3c/screen-orientation#232

Differential Revision: https://phabricator.services.mozilla.com/D168349

modules/libpref/init/StaticPrefList.yaml

@@ -3498,7 +3498,7 @@
 # Enable Screen Orientation lock
 - name: dom.screenorientation.allow-lock
   type: bool
-  value: false
+  value: @IS_NIGHTLY_BUILD@
   mirror: always
 
 # Whether to enable the JavaScript start-up cache. This causes one of the first