[TabMetrics] Add signals that mark the start and end of session restore.

chrome/browser/sessions/tab_loader.cc

Index: chrome/browser/sessions/tab_loader.cc
diff --git a/chrome/browser/sessions/tab_loader.cc b/chrome/browser/sessions/tab_loader.cc
index 753f4e0493c23ad66dc15e7f1f37a6574ce3f7e1..825a2a19eca98725d54daba751db8dcbbaf61f68 100644
--- a/chrome/browser/sessions/tab_loader.cc
+++ b/chrome/browser/sessions/tab_loader.cc
@@ -13,6 +13,8 @@
 #include "base/memory/ptr_util.h"
 #include "base/metrics/histogram_macros.h"
 #include "build/build_config.h"
+#include "chrome/browser/browser_process.h"
+#include "chrome/browser/resource_coordinator/tab_manager.h"
 #include "chrome/browser/sessions/session_restore_stats_collector.h"
 #include "chrome/browser/ui/browser.h"
 #include "chrome/browser/ui/browser_finder.h"
@@ -103,6 +105,9 @@ TabLoader::TabLoader(base::TimeTicks restore_started)
   shared_tab_loader_ = this;
   this_retainer_ = this;
   base::MemoryCoordinatorClientRegistry::GetInstance()->Register(this);
+
+  DCHECK(g_browser_process->GetTabManager());
+  g_browser_process->GetTabManager()->OnSessionRestoreStarted();

[sky, 2017/06/21 00:40:36]

Putting this here isn't entirely correct as TabLoader is created *after* session
restore has waited to read the file.

I also think other places may be interested in tracking the lifetime of session
restore so that I would prefer a generic observer interface. TabLoader shouldn't
have a direct dependency on TabManager, it should only care about notifying
observers.

 }
 
 TabLoader::~TabLoader() {
@@ -110,6 +115,9 @@ TabLoader::~TabLoader() {
   DCHECK(shared_tab_loader_ == this);
   shared_tab_loader_ = nullptr;
   base::MemoryCoordinatorClientRegistry::GetInstance()->Unregister(this);
+
+  DCHECK(g_browser_process->GetTabManager());
+  g_browser_process->GetTabManager()->OnSessionRestoreEnded();
 }
 
 void TabLoader::StartLoading(const std::vector<RestoredTab>& tabs) {