Bug 1641929 - Add a fission.autostart getter, r=nika

This makes it easier to consider special conditions when checking the pref. It
currently returns false if safe mode is enabled.

Differential Revision: https://phabricator.services.mozilla.com/D78798

toolkit/xre/nsAppRunner.cpp

@@ -24,6 +24,7 @@
 #include "mozilla/ResultExtensions.h"
 #include "mozilla/ScopeExit.h"
 #include "mozilla/Services.h"
+#include "mozilla/StaticPrefs_fission.h"
 #include "mozilla/Telemetry.h"
 #include "mozilla/Utf8.h"
 #include "mozilla/intl/LocaleService.h"
@@ -5054,6 +5055,10 @@ bool BrowserTabsRemoteAutostart() {
   return gBrowserTabsRemoteAutostart;
 }
 
+bool FissionAutostart() {
+  return !gSafeMode && StaticPrefs::fission_autostart();
+}
+
 uint32_t GetMaxWebProcessCount() {
   // multiOptOut is in int to allow us to run multiple experiments without
   // introducing multiple prefs a la the autostart.N prefs.

xpcom/system/nsIXULRuntime.idl

@@ -13,6 +13,13 @@ namespace mozilla {
 // therefore be disabled. Bug 1065561 tracks its removal.
 bool BrowserTabsRemoteAutostart();
 uint32_t GetMaxWebProcessCount();
+
+// Returns the value of the fission.autostart pref. Since fission can be
+// disabled on a per-window basis, this should only be used when you need the
+// global value of the pref. For other use cases, you should use
+// nsILoadContext::UseRemoteSubframes instead. This will also check for special
+// conditions, like safe mode, which may require fission to be disabled.
+bool FissionAutostart();
 }
 
 %}