[Sessions] Add detailed logging of SessionRestore events.

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 df77a8c906ede3d188c1aad804953c1fb78ba162..ce0c0a4b466b5addb46e071bcd966d8b23c9a146 100644
--- a/chrome/browser/sessions/tab_loader.cc
+++ b/chrome/browser/sessions/tab_loader.cc
@@ -7,6 +7,7 @@
 #include <algorithm>
 #include <string>
 
+#include "base/metrics/field_trial.h"
 #include "base/metrics/histogram.h"
 #include "base/strings/stringprintf.h"
 #include "chrome/browser/ui/browser.h"
@@ -17,6 +18,7 @@
 #include "content/public/browser/notification_types.h"
 #include "content/public/browser/render_widget_host.h"
 #include "content/public/browser/render_widget_host_view.h"
+#include "content/public/browser/session_restore_uma.h"
 #include "content/public/browser/web_contents.h"
 
 using content::NavigationController;
@@ -117,6 +119,17 @@ void TabLoader::LoadNextTab() {
     DCHECK(controller);
     tabs_loading_.insert(controller);
     tabs_to_load_.pop_front();
+
+    // Keep track of tabs that have been automatically loaded by session
+    // restore. This is the complement to SessionRestore.DeferredTabLoadedByUser
+    // recorded in NavigationControllerImpl::SetActive.
+    if (controller->NeedsReload()) {
+      UMA_HISTOGRAM_ENUMERATION(
+          content::kSessionRestoreTabActions,
+          content::kSessionRestoreTabActionsUma_BackgroundTabLoaded,
+          content::kSessionRestoreTabActionsUma_Max);
+    }
+
     controller->LoadIfNecessary();
     content::WebContents* contents = controller->GetWebContents();
     if (contents) {
@@ -183,6 +196,21 @@ void TabLoader::HandleTabClosedOrLoaded(NavigationController* controller) {
 
 void TabLoader::OnMemoryPressure(
     base::MemoryPressureListener::MemoryPressureLevel memory_pressure_level) {
+  // On Windows and Mac this mechanism is only experimentally enabled.
+#if defined(OS_WIN) or (defined(OS_MACOSX) and !defined(OS_IOS))
+  // If memory pressure isn't explicitly turned on then ignore these calls.
+  std::string react_to_memory_pressure = variations::GetVariationParamValue(
+      kSessionRestoreBackgroundLoading, "ReactToMemoryPressure"));
+  if (react_to_memory_pressure != "1")
+    return;
+#endif
+
+  // Note that the session restore was interrupted by memory pressure.
+  UMA_HISTOGRAM_ENUMERATION(
+          content::kSessionRestoreActions,
+          content::kSessionRestoreActionsUma_InterruptedByMemoryPressure,
+          content::kSessionRestoreActionsUma_Max);
+
   // When receiving a resource pressure level warning, we stop pre-loading more
   // tabs since we are running in danger of loading more tabs by throwing out
   // old ones.
@@ -191,6 +219,12 @@ void TabLoader::OnMemoryPressure(
   // Stop the timer and suppress any tab loads while we clean the list.
   SetTabLoadingEnabled(false);
   while (!tabs_to_load_.empty()) {
+    // Count the number of tabs that have had their loading deferred.
+    UMA_HISTOGRAM_ENUMERATION(
+        content::kSessionRestoreTabActions,
+        content::kSessionRestoreTabActionsUma_TabLoadingDeferred,
+        content::kSessionRestoreTabActionsUma_Max);
+
     NavigationController* controller = tabs_to_load_.front();
     tabs_to_load_.pop_front();
     RemoveTab(controller);