Bug 1843044 - Add a missing include and while at it dynamic handling for the titlebar color pref.

MANUAL PUSH: Trivial fix CLOSED TREE

widget/nsXPLookAndFeel.cpp

@@ -486,12 +486,13 @@ static constexpr struct {
     // Affects whether standins are used for the accent color.
     {"widget.non-native-theme.use-theme-accent"_ns,
      widget::ThemeChangeKind::Style},
-    // These two affect system colors on Windows.
+    // These three affect system colors on Windows.
     {"widget.windows.uwp-system-colors.enabled"_ns,
      widget::ThemeChangeKind::Style},
-    // These two affect system colors on Windows.
     {"widget.windows.uwp-system-colors.highlight-accent"_ns,
      widget::ThemeChangeKind::Style},
+    {"widget.windows.titlebar-accent.enabled"_ns,
+     widget::ThemeChangeKind::Style},
     // Affects env().
     {"layout.css.prefers-color-scheme.content-override"_ns,
      widget::ThemeChangeKind::Style},

widget/windows/nsLookAndFeel.cpp

@@ -17,6 +17,7 @@
 #include "mozilla/Telemetry.h"
 #include "gfxFontConstants.h"
 #include "gfxWindowsPlatform.h"
+#include "mozilla/StaticPrefs_widget.h"
 
 using namespace mozilla;
 using namespace mozilla::widget;