[Session restore] Expose lazy mode and fix browser tests.

- SessionRestore now exposes IsLoadingActiveTabsOnly
- Fixed 2 browser tests that were failing with new behavior
- One last telemetry test still failing, will be addressed in a separate CL

Notes:
- First patchset inverts the default behavior to show tests succeeding
- Second patchset restores default behavior

BUG=

Review URL: https://codereview.chromium.org/1087063003

Cr-Commit-Position: refs/heads/master@{#325528}

chrome/browser/sessions/better_session_restore_browsertest.cc

@@ -214,6 +214,14 @@ class BetterSessionRestoreTest : public InProcessBrowserTest {
   }
 
   void CheckReloadedPageNotRestored() {
+    // If only active tabs are loaded, force loading of the tabs.
+    if (SessionRestore::WillLoadActiveTabsOnly()) {
+      for (int i = 0; i < browser()->tab_strip_model()->count(); i++) {
+        content::WebContents* web_contents =
+            browser()->tab_strip_model()->GetWebContentsAt(i);
+        web_contents->GetController().LoadIfNecessary();
+      }
+    }
     CheckReloadedPageNotRestored(browser());
   }
 

chrome/browser/sessions/session_restore.cc

@@ -16,6 +16,7 @@
 #include "base/debug/alias.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/memory/scoped_vector.h"
+#include "base/metrics/field_trial.h"
 #include "base/metrics/histogram.h"
 #include "base/run_loop.h"
 #include "base/stl_util.h"
@@ -298,7 +299,9 @@ class SessionRestoreImpl : public content::NotificationObserver {
 
     if (succeeded) {
       // Start Loading tabs.
-      SessionRestoreDelegate::RestoreTabs(contents_created, restore_started_);
+      bool active_only = SessionRestore::WillLoadActiveTabsOnly();
+      SessionRestoreDelegate::RestoreTabs(contents_created, restore_started_,
+                                          active_only);
     }
 
     if (!synchronous_) {
@@ -817,6 +820,15 @@ SessionRestore::CallbackSubscription
   return on_session_restored_callbacks()->Add(callback);
 }
 
+// static
+bool SessionRestore::WillLoadActiveTabsOnly() {
+  base::FieldTrial* trial =
+      base::FieldTrialList::Find("SessionRestoreBackgroundLoading");
+  if (!trial || trial->group_name() == "Restore")
+    return false;
+  return true;
+}
+
 // static
 base::CallbackList<void(int)>*
     SessionRestore::on_session_restored_callbacks_ = nullptr;

chrome/browser/sessions/session_restore.h

@@ -98,6 +98,11 @@ class SessionRestore {
   static CallbackSubscription RegisterOnSessionRestoredCallback(
       const base::Callback<void(int)>& callback);
 
+  // Returns true if only active tabs are to be loaded during session restore
+  // (lazy load). Otherwise returns false if all tabs will be loaded when
+  // restored.
+  static bool WillLoadActiveTabsOnly();
+
  private:
   SessionRestore();
 

chrome/browser/sessions/session_restore_delegate.cc

@@ -10,17 +10,12 @@
 #include "components/favicon/content/content_favicon_driver.h"
 
 // static
-void SessionRestoreDelegate::RestoreTabs(
-    const std::vector<RestoredTab>& tabs,
-    const base::TimeTicks& restore_started) {
-  // TODO(georgesak): make tests aware of that behavior so that they succeed if
-  // tab loading is disabled.
-  base::FieldTrial* trial =
-      base::FieldTrialList::Find("SessionRestoreBackgroundLoading");
-  bool active_only = true;
-  if (!trial || trial->group_name() == "Restore") {
+void SessionRestoreDelegate::RestoreTabs(const std::vector<RestoredTab>& tabs,
+                                         const base::TimeTicks& restore_started,
+                                         bool active_only) {
+  SessionRestoreStatsCollector::TrackTabs(tabs, restore_started, active_only);
+  if (!active_only) {
     TabLoader::RestoreTabs(tabs, restore_started);
-    active_only = false;
   } else {
     // If we are not loading inactive tabs, restore their favicons (title has
     // already been set by now).
@@ -32,5 +27,4 @@ void SessionRestoreDelegate::RestoreTabs(
       }
     }
   }
-  SessionRestoreStatsCollector::TrackTabs(tabs, restore_started, active_only);
 }

chrome/browser/sessions/session_restore_delegate.h

@@ -23,7 +23,8 @@ class SessionRestoreDelegate {
   };
 
   static void RestoreTabs(const std::vector<RestoredTab>& tabs,
-                          const base::TimeTicks& restore_started);
+                          const base::TimeTicks& restore_started,
+                          bool active_only);
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(SessionRestoreDelegate);