Update AuraWindowRegistry to create only positive window IDs.

Previously AuraWindowRegistry was allocating ID 0 to first window. This
breaks assumptions made in other places, e.g. DCHECK in
DesktopMediaPickerDialogView. After this change the implementation
guarantees that window IDs are always unique positive integers.

Review URL: https://codereview.chromium.org/499813002

Cr-Commit-Position: refs/heads/master@{#291708}

content/public/browser/desktop_media_id.cc

@@ -32,7 +32,12 @@ class AuraWindowRegistry : public aura::WindowObserver {
     }
 
     // If the windows doesn't have an Id yet assign it.
-    int id = next_id_++;
+    int id;
+    do {
+      id = next_id_;
+      next_id_ = (next_id_ == INT_MAX) ? 1 : (next_id_ + 1);
+    } while (id_to_window_map_.find(id) != id_to_window_map_.end());
+
     window_to_id_map_[window] = id;
     id_to_window_map_[id] = window;
     window->AddObserver(this);
@@ -48,7 +53,7 @@ class AuraWindowRegistry : public aura::WindowObserver {
   friend struct DefaultSingletonTraits<AuraWindowRegistry>;
 
   AuraWindowRegistry()
-      : next_id_(0) {
+      : next_id_(1) {
   }
   virtual ~AuraWindowRegistry() {}