Add a "session only" policy to the content settings.

chrome/browser/cocoa/content_exceptions_window_controller.mm

Index: chrome/browser/cocoa/content_exceptions_window_controller.mm
diff --git a/chrome/browser/cocoa/content_exceptions_window_controller.mm b/chrome/browser/cocoa/content_exceptions_window_controller.mm
index 0ebcb8ec9b735b7df90454d1e023bcead1dad2c4..5581153881bbd690fb99f950607b035e72fb1a20 100644
--- a/chrome/browser/cocoa/content_exceptions_window_controller.mm
+++ b/chrome/browser/cocoa/content_exceptions_window_controller.mm
@@ -128,6 +128,7 @@ const ContentSetting kNoAskSettings[] = { CONTENT_SETTING_ALLOW,
 // The settings shown in the combobox if showAsk_ is true;
 const ContentSetting kAskSettings[] = { CONTENT_SETTING_ALLOW,
                                         CONTENT_SETTING_ASK,
+                                        CONTENT_SETTING_SESSION_ONLY,
                                         CONTENT_SETTING_BLOCK };
 
 }  // namespace
@@ -476,6 +477,9 @@ static ContentExceptionsWindowController*
       return l10n_util::GetNSStringWithFixup(IDS_EXCEPTIONS_BLOCK_BUTTON);
     case CONTENT_SETTING_ASK:
       return l10n_util::GetNSStringWithFixup(IDS_EXCEPTIONS_ASK_BUTTON);
+    case CONTENT_SETTING_SESSION_ONLY:
+      return l10n_util::GetNSStringWithFixup(
+          IDS_EXCEPTIONS_SESSION_ONLY_BUTTON);
     default:
       NOTREACHED();
   }