[ios] Expose TabModel underlying WebStateList.

ios/chrome/browser/tabs/tab_model.h

Index: ios/chrome/browser/tabs/tab_model.h
diff --git a/ios/chrome/browser/tabs/tab_model.h b/ios/chrome/browser/tabs/tab_model.h
index d842d2c9cbe06c47132b26d2eef474ef0f4dade4..c7e20413c70a472089b9722da6129cbe077c3f03 100644
--- a/ios/chrome/browser/tabs/tab_model.h
+++ b/ios/chrome/browser/tabs/tab_model.h
@@ -21,6 +21,7 @@ class SessionID;
 @protocol TabModelObserver;
 class TabModelSyncedWindowDelegate;
 class TabUsageRecorder;
+class WebStateList;
 
 namespace ios {
 class ChromeBrowserState;
@@ -113,6 +114,9 @@ NSUInteger const kTabPositionAutomatically = NSNotFound;
 // Determines the number of tabs in the model.
 @property(nonatomic, readonly) NSUInteger count;
 
+// The WebStateList owned by the TabModel.
+@property(nonatomic, readonly) WebStateList* webStateList;
+
 // Initializes tabs from a restored session. |-setCurrentTab| needs to be called
 // in order to display the views associated with the tabs. Waits until the views
 // are ready. |browserState| cannot be nil. |service| cannot be nil; this class