Fix tab discarding reload count metric.

chrome/browser/ui/tabs/tab_discard_state.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/ui/tabs/tab_discard_state.h"
 
+#include "base/metrics/histogram.h"
+#include "content/public/browser/browser_thread.h"
 #include "content/public/browser/web_contents.h"
 
 using content::WebContents;
 
 namespace {
 
 const char kDiscardStateKey[] = "TabDiscardState";
 
 }  // namespace
 
 // static
 TabDiscardState* TabDiscardState::Get(WebContents* web_contents) {
+  DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
   TabDiscardState* discard_state = static_cast<TabDiscardState*>(
       web_contents->GetUserData(&kDiscardStateKey));
 
   // If this function is called, we probably need to query/change the discard
   // state. Let's go ahead a add one.
   if (!discard_state) {
     discard_state = new TabDiscardState;
     web_contents->SetUserData(&kDiscardStateKey, discard_state);
   }
 
   return discard_state;
 }
 
 // static
 void TabDiscardState::Set(WebContents* web_contents, TabDiscardState* state) {
+  DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
   web_contents->SetUserData(&kDiscardStateKey, state);
 }
 
 // static
 bool TabDiscardState::IsDiscarded(WebContents* web_contents) {
   TabDiscardState* discard_state = TabDiscardState::Get(web_contents);
   return discard_state->is_discarded_;
 }
 
 // static
 void TabDiscardState::SetDiscardState(WebContents* web_contents, bool state) {
   TabDiscardState* discard_state = TabDiscardState::Get(web_contents);
+  if (discard_state->is_discarded_ && !state) {
+    static int reload_count = 0;
+    UMA_HISTOGRAM_CUSTOM_COUNTS("Tabs.Discard.ReloadCount", ++reload_count, 1,
+                                1000, 50);
+  }
+
   discard_state->is_discarded_ = state;
 }
 
 // static
 int TabDiscardState::DiscardCount(WebContents* web_contents) {
   TabDiscardState* discard_state = TabDiscardState::Get(web_contents);
   return discard_state->discard_count_;
 }
 
 // static
 void TabDiscardState::IncrementDiscardCount(WebContents* web_contents) {
   TabDiscardState* discard_state = TabDiscardState::Get(web_contents);
   discard_state->discard_count_++;
 }
 
 // static
 void TabDiscardState::CopyState(content::WebContents* old_contents,
                                 content::WebContents* new_contents) {
   TabDiscardState* old_state = Get(old_contents);
   TabDiscardState* new_State = Get(new_contents);
   *new_State = *old_state;
 }