Last_n: Add instrumentation test for the tab closure case.

chrome/browser/android/offline_pages/recent_tab_helper.cc

Index: chrome/browser/android/offline_pages/recent_tab_helper.cc
diff --git a/chrome/browser/android/offline_pages/recent_tab_helper.cc b/chrome/browser/android/offline_pages/recent_tab_helper.cc
index 191754d8f56833e26d8c2f413394db8cecaa924f..01c2011e17a8c6766b0d4c0224837d5049eed7f4 100644
--- a/chrome/browser/android/offline_pages/recent_tab_helper.cc
+++ b/chrome/browser/android/offline_pages/recent_tab_helper.cc
@@ -261,9 +261,6 @@ void RecentTabHelper::WebContentsDestroyed() {
   CancelInFlightSnapshots();
 }
 
-// TODO(carlosk): this method is also called when the tab is being closed, when
-// saving a snapshot is probably useless (low probability of the user undoing
-// the close). We should detect that and avoid the saving.
 void RecentTabHelper::WasHidden() {
   if (!IsOffliningRecentPagesEnabled())
     return;