win: Let CalculateIdleState() deal better with GetTickCount() wrapping around.

chrome/browser/idle_win.cc

Index: chrome/browser/idle_win.cc
diff --git a/chrome/browser/idle_win.cc b/chrome/browser/idle_win.cc
index 01afca083998508b41f15277970d008063ec1383..712211943e107a2d69e9265787d18a70a87b7900 100644
--- a/chrome/browser/idle_win.cc
+++ b/chrome/browser/idle_win.cc
@@ -18,10 +18,19 @@ IdleState CalculateIdleState(unsigned int idle_threshold) {
   last_input_info.cbSize = sizeof(LASTINPUTINFO);
   DWORD current_idle_time = 0;
   if (::GetLastInputInfo(&last_input_info)) {
-    current_idle_time = ::GetTickCount() - last_input_info.dwTime;
-    // Will go -ve if we have been idle for a long time (2gb seconds).
-    if (current_idle_time < 0)
-      current_idle_time = INT_MAX;
+    DWORD now = ::GetTickCount();
+    if (now < last_input_info.dwTime) {
+      // GetTickCount() wraps around every 49.7 days -- assume it wrapped just
+      // once.
+      const DWORD kMaxDWORD = static_cast<DWORD>(-1);
+      DWORD time_before_wrap = kMaxDWORD - last_input_info.dwTime;
+      DWORD time_after_wrap = now;
+      // The sum is always smaller than kMaxDWORD.
+      current_idle_time = time_before_wrap + time_after_wrap;
+    } else {
+      current_idle_time = now - last_input_info.dwTime;
+    }
+
     // Convert from ms to seconds.
     current_idle_time /= 1000;
   }