Bug 1697607 - Enable widget.remote-look-and-feel everywhere. r=cmartin,geckoview-reviewers,agi

(And add a missing include while at it)

The biggest concern here is potential startup time penalty (and it
shouldn't be much anyways, if at all). In exchange, we avoid doing a lot
of this work in content processes.

Let's keep an eye on for regressions, but this sticking allows us to
simplify a lot of the lookandfeel code in follow-ups.

Differential Revision: https://phabricator.services.mozilla.com/D107926

modules/libpref/init/StaticPrefList.yaml

@@ -10730,11 +10730,7 @@
 # correctly when `security.sandbox.content.headless` is enabled.
 - name: widget.remote-look-and-feel
   type: bool
-#ifdef MOZ_WIDGET_GTK
   value: true
-#else
-  value: false
-#endif
   mirror: once
 
 #---------------------------------------------------------------------------

widget/RemoteLookAndFeel.cpp

@@ -10,6 +10,7 @@
 #include "gfxFont.h"
 #include "MainThreadUtils.h"
 #include "mozilla/Assertions.h"
+#include "mozilla/ClearOnShutdown.h"
 #include "mozilla/Result.h"
 #include "mozilla/ResultExtensions.h"
 #include "mozilla/StaticPrefs_widget.h"