This is just an idea for how to implement idle_timer which isn't yet feasible...

base/idle_timer.cc

Index: base/idle_timer.cc
===================================================================
--- base/idle_timer.cc	(revision 2086)
+++ base/idle_timer.cc	(working copy)
@@ -9,10 +9,59 @@
 
 namespace base {
 
+#if defined(OS_LINUX)
+struct IdleTimer::XssInfoLazyInstanceTraits {
+  static void New(void* instance) {
+    XssInfo* xss_info = static_cast<XssInfo>(instance);
+    int event_base, error_base;
+    xss_info->have_xss = XScreenSaverQueryExtension(GDK_DISPLAY(), &event_base,
+                                                    &error_base);
+    if (xss_info->have_xss)
+      xss_info->xss_info.Get() = XScreenSaverAllocInfo();
+  }
+  static void Delete(void* instance) {
+    if (xss_info->xss_info.Get()) {
+      XFree(xss_info->xss_info.Get());
+      xss_info->xss_info.~ThreadLocalPointer();
+    }
+  }
+};
+#endif
+
+// static
+bool IdleTimer::DefaultIdleTimeFunction(int32* milliseconds)
+{
+#if defined(OS_WIN)
+  LASTINPUTINFO info;
+  info.cbSize = sizeof(info);
+  if (GetLastInputInfo(&info)) {
+    int32 last_input_time = info.dwTime;
+    int32 current_time = GetTickCount();
+    int32 interval = current_time - last_input_time;
+    // Interval will go negative if we've been idle for 2GB of ticks.
+    if (interval < 0)
+      interval = -interval;
+    *milliseconds = interval;
+    return true;
+  }
+  return false;
+#elif defined(OS_LINUX)
+  static LazyInstance<XssInfo, XssInfoLazyInstanceTraits> xss;
+  XssInfo* xss_info = xss.Pointer();
+  if (xss_info->have_xss && xss_info->xss_info.Get()) {
+    XScreenSaverQueryInfo(GDK_DISPLAY(), GDK_ROOT_WINDOW(),
+                          xss_info->xss_info.Get());
+    *milliseconds = xss_info->xss_info.Get()->idle;
+    return true;
+  }
+  return false;
+#endif
+}
+
 IdleTimer::IdleTimer(TimeDelta idle_time, bool repeat)
     : idle_interval_(idle_time),
       repeat_(repeat),
-      get_last_input_info_fn_(GetLastInputInfo) {
+      get_idle_time_fn_(GetLastInputInfo) {
   DCHECK_EQ(MessageLoop::TYPE_UI, MessageLoop::current()->type()) <<
       "Requires a thread that processes Windows UI events";
 }