aura: Have aura and aura_demo build and run without gtk on linux.

ui/base/x/x11_util.h

Index: ui/base/x/x11_util.h
diff --git a/ui/base/x/x11_util.h b/ui/base/x/x11_util.h
index fb0304e6824433085c81e54c35e58a6e46791cf3..6763c04a451bc68846e44e05ccf4af100952875f 100644
--- a/ui/base/x/x11_util.h
+++ b/ui/base/x/x11_util.h
@@ -19,13 +19,16 @@
 #include "ui/base/ui_export.h"
 
 typedef unsigned long Atom;
-typedef struct _GdkDrawable GdkWindow;
-typedef struct _GtkWidget GtkWidget;
-typedef struct _GtkWindow GtkWindow;
 typedef unsigned long XID;
 typedef unsigned long XSharedMemoryId;  // ShmSeg in the X headers.
 typedef struct _XDisplay Display;
 
+#if defined(TOOLKIT_USES_GTK)
+typedef struct _GdkDrawable GdkWindow;
+typedef struct _GtkWidget GtkWidget;
+typedef struct _GtkWindow GtkWindow;
+#endif
+
 namespace gfx {
 class Rect;
 }
@@ -72,6 +75,7 @@ UI_EXPORT XID GetX11RootWindow();
 // Returns the user's current desktop.
 bool GetCurrentDesktop(int* desktop);
 
+#if defined(TOOLKIT_USES_GTK)
 // Get the X window id for the given GTK widget.
 UI_EXPORT XID GetX11WindowFromGtkWidget(GtkWidget* widget);
 XID GetX11WindowFromGdkWindow(GdkWindow* window);
@@ -84,6 +88,7 @@ UI_EXPORT GtkWindow* GetGtkWindowFromX11Window(XID xid);
 // Get a Visual from the given widget. Since we don't include the Xlib
 // headers, this is returned as a void*.
 UI_EXPORT void* GetVisualFromGtkWidget(GtkWidget* widget);
+#endif  // defined(TOOLKIT_USES_GTK)
 
 // Return the number of bits-per-pixel for a pixmap of the given depth
 UI_EXPORT int BitsPerPixelForPixmapDepth(Display* display, int depth);