Bug 1574090 - Make fission.autostart available via StaticPrefs. r=njn

Differential Revision: https://phabricator.services.mozilla.com/D42091

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

modules/libpref/init/StaticPrefList.yaml

@@ -2566,6 +2566,13 @@
 # Prefs starting with "fission."
 #---------------------------------------------------------------------------
 
+# Whether to enable Fission.
+# Overridden in all.js on RELEASE_OR_BETA in order to add the locked attribute.
+- name: fission.autostart
+  type: bool
+  value: false
+  mirror: always
+
 # This pref has no effect within fission windows, it only controls the
 # behaviour within non-fission windows.
 - name: fission.preserve_browsing_contexts

toolkit/components/windowwatcher/nsWindowWatcher.cpp

@@ -62,6 +62,7 @@
 #include "mozilla/NullPrincipal.h"
 #include "mozilla/Preferences.h"
 #include "mozilla/ResultExtensions.h"
+#include "mozilla/StaticPrefs_fission.h"
 #include "mozilla/dom/Element.h"
 #include "mozilla/dom/Storage.h"
 #include "mozilla/dom/ScriptSettings.h"
@@ -474,7 +475,7 @@ nsWindowWatcher::OpenWindowWithRemoteTab(
     return NS_ERROR_UNEXPECTED;
   }
 
-  bool isFissionWindow = Preferences::GetBool("fission.autostart");
+  bool isFissionWindow = StaticPrefs::fission_autostart();
   bool isPrivateBrowsingWindow =
       Preferences::GetBool("browser.privatebrowsing.autostart");
 
@@ -1803,7 +1804,7 @@ uint32_t nsWindowWatcher::CalculateChromeFlagsForParent(
   }
 
   // Determine whether the window should have remote subframes
-  bool fission = Preferences::GetBool("fission.autostart");
+  bool fission = StaticPrefs::fission_autostart();
 
   if (fission) {
     fission = !WinHasOption(aFeatures, "non-fission", 0, &presenceFlag);

xpfe/appshell/nsAppShellService.cpp

@@ -40,6 +40,7 @@
 #include "mozilla/Preferences.h"
 #include "mozilla/Services.h"
 #include "mozilla/StartupTimeline.h"
+#include "mozilla/StaticPrefs_fission.h"
 #include "mozilla/intl/LocaleService.h"
 
 #include "nsEmbedCID.h"
@@ -704,7 +705,7 @@ nsresult nsAppShellService::JustCreateTopWindow(
   bool isPrivateBrowsingWindow =
       Preferences::GetBool("browser.privatebrowsing.autostart");
   bool isUsingRemoteTabs = mozilla::BrowserTabsRemoteAutostart();
-  bool isUsingRemoteSubframes = Preferences::GetBool("fission.autostart");
+  bool isUsingRemoteSubframes = StaticPrefs::fission_autostart();
 
   if (aChromeMask & nsIWebBrowserChrome::CHROME_PRIVATE_WINDOW) {
     // Caller requested a private window