Add histogram to track prerender sessions

chrome/browser/prerender/prerender_manager.h

Index: chrome/browser/prerender/prerender_manager.h
diff --git a/chrome/browser/prerender/prerender_manager.h b/chrome/browser/prerender/prerender_manager.h
index c4a6375ab7b01ccb374c4ef334de2f29fa21c4dc..d5c6bdb975c195e27c7561a6716a64f0f5af177b 100644
--- a/chrome/browser/prerender/prerender_manager.h
+++ b/chrome/browser/prerender/prerender_manager.h
@@ -24,6 +24,14 @@ class TabContents;
 class PrerenderManager : public base::RefCounted<PrerenderManager>,
                          private base::NonThreadSafe {
  public:
+  // SessionType is used in a UMA_HISTOGRAM, so please do not add in the
+  // middle.
+  enum SessionType {
+    PRERENDERING_OFF,
+    PRERENDERING_ON,
+    PRERENDERING_SESSION_TYPE_MAX
+  };
+
   // Owned by a Profile object for the lifetime of the profile.
   explicit PrerenderManager(Profile* profile);
 
@@ -52,6 +60,8 @@ class PrerenderManager : public base::RefCounted<PrerenderManager>,
   unsigned int max_elements() const { return max_elements_; }
   void set_max_elements(unsigned int num) { max_elements_ = num; }
 
+  static SessionType GetSessionType();
+
  protected:
   virtual ~PrerenderManager();