Bug 1521934 - Set devtools.responsive.metaViewport.enabled pref to true; r=bradwerth

Now that all of the important metaViewport simulation bugs have been fixed, we would
like to let feature ride the trains, enabled by default.

This means we can also stop setting the pref in tests that rely on it.

Differential Revision: https://phabricator.services.mozilla.com/D53005

--HG--
extra : moz-landing-system : lando

browser/app/profile/firefox.js

@@ -2284,7 +2284,14 @@ pref("devtools.responsive.reloadNotification.enabled", true);
 pref("devtools.responsive.touchSimulation.enabled", false);
 // Whether or not meta viewport is enabled, if and only if touchSimulation
 // is also enabled.
-pref("devtools.responsive.metaViewport.enabled", false);
+// For now this is only available in nightly, dev-edition and early betas. It is planned
+// to be gradually rolled out with release 72. Starting with 73, this pref needs to be set
+// to true on all channels.
+#if defined(EARLY_BETA_OR_EARLIER) || defined(MOZ_DEV_EDITION)
+  pref("devtools.responsive.metaViewport.enabled", true);
+#else
+  pref("devtools.responsive.metaViewport.enabled", false);
+#endif
 // The user agent of the viewport.
 pref("devtools.responsive.userAgent", "");
 // Whether or not the RDM UI is embedded in the browser.