Aura: Combine cursor support for Linux and Windows

+ Change gfx::NativeCursor to an int id on Aura.
+ Fix several cursor-related TODOs in GetCursor() methods.
+ Move X11 cursor cache into x11_util
+ Set platform-specific cursors in aura::DesktopHostWin and DesktopHostLinux

BUG=99871
TEST=manual

Review URL: http://codereview.chromium.org/8256001

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@105850 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/ui/aura/demo/demo_main.cc b/ui/aura/demo/demo_main.cc
index 335b298..7522fba5 100644
--- a/ui/aura/demo/demo_main.cc
+++ b/ui/aura/demo/demo_main.cc
@@ -38,7 +38,7 @@
     return false;
   }
   virtual gfx::NativeCursor GetCursor(const gfx::Point& point) OVERRIDE {
-    return NULL;
+    return gfx::kNullCursor;
   }
   virtual int GetNonClientComponent(const gfx::Point& point) const OVERRIDE {
     return HTCAPTION;