Allow dynamic switching in and out of sidetabs mode.

chrome/browser/browser.h

Index: chrome/browser/browser.h
===================================================================
--- chrome/browser/browser.h	(revision 41630)
+++ chrome/browser/browser.h	(working copy)
@@ -575,6 +575,8 @@
   virtual bool CanCloseContentsAt(int index);
   virtual bool CanBookmarkAllTabs() const;
   virtual void BookmarkAllTabs();
+  virtual bool UseVerticalTabs() const;
+  virtual void ToggleUseVerticalTabs();
 
   // Overridden from TabStripModelObserver:
   virtual void TabInsertedAt(TabContents* contents,
@@ -915,6 +917,9 @@
   // from a TabContents. Currently, only one pending action is allowed.
   WebAppAction pending_web_app_action_;
 
+  // Tracks the display mode of the tabstrip.
+  mutable BooleanPrefMember use_vertical_tabs_;
+
   DISALLOW_COPY_AND_ASSIGN(Browser);
 };