Restart counters when the MD CBD dialog is reopened.

chrome/browser/ui/webui/settings/settings_clear_browsing_data_handler.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/webui/settings/settings_clear_browsing_data_handler.h"
 
 #include <stddef.h>
 
 #include "base/macros.h"
 #include "base/memory/ptr_util.h"
@@ skipping 254 matching lines @@
   AllowJavascript();
   const base::Value* callback_id;
   CHECK(args->Get(0, &callback_id));
 
   // Needed because WebUI doesn't handle renderer crashes. See crbug.com/610450.
   task_observer_.reset();
 
   OnStateChanged();
   RefreshHistoryNotice();
 
+  // Restart the counters if the dialog is reopened.
+  for (const auto& counter : counters_)
+    counter->Restart();

[tommycli, 2016/12/16 17:39:14]

how much performance penalty is it to create the counter in AllowJavascript and
then immediately restart it here?

Do counters kick off any I/O operations on a separate thread for instance?

[msramek, 2017/01/03 13:56:39]

Oops, I didn't realize that AllowJavascript() is called in HandleInitialize()
*before* we do the restart. That would be indeed a duplication. Removing
Restart() from AddCounter() should do the trick. In the last patchset, Restart()
is called exactly once for each counter each time the dialog is opened.

And yes, counting is done asynchronously, for example reading the cache index on
the IO thread, or the passwords database on the DB thread.

+
   ResolveJavascriptCallback(
       *callback_id,
       *base::Value::CreateNullValue() /* Promise<void> */);
 }
 
 void ClearBrowsingDataHandler::OnStateChanged() {
   CallJavascriptFunction(
       "cr.webUIListenerCallback",
       base::StringValue("update-footer"),
       base::FundamentalValue(sync_service_ && sync_service_->IsSyncActive()),
@@ skipping 49 matching lines @@
 void ClearBrowsingDataHandler::UpdateCounterText(
     std::unique_ptr<browsing_data::BrowsingDataCounter::Result> result) {
   CallJavascriptFunction(
       "cr.webUIListenerCallback",
       base::StringValue("update-counter-text"),
       base::StringValue(result->source()->GetPrefName()),
       base::StringValue(GetChromeCounterTextFromResult(result.get())));
 }
 
 }  // namespace settings