[Tab Metrics] Measure FP, FCP and FMP for Foreground Tab during Session Restore

chrome/browser/page_load_metrics/page_load_metrics_initialize.cc

Index: chrome/browser/page_load_metrics/page_load_metrics_initialize.cc
diff --git a/chrome/browser/page_load_metrics/page_load_metrics_initialize.cc b/chrome/browser/page_load_metrics/page_load_metrics_initialize.cc
index 4e400fbd206f4ea2c8ea37c521fd76d762a86e76..28deec47612ea7ec24b9a0179e6efd02c3260897 100644
--- a/chrome/browser/page_load_metrics/page_load_metrics_initialize.cc
+++ b/chrome/browser/page_load_metrics/page_load_metrics_initialize.cc
@@ -34,6 +34,7 @@
 #include "chrome/browser/page_load_metrics/observers/previews_page_load_metrics_observer.h"
 #include "chrome/browser/page_load_metrics/observers/protocol_page_load_metrics_observer.h"
 #include "chrome/browser/page_load_metrics/observers/service_worker_page_load_metrics_observer.h"
+#include "chrome/browser/page_load_metrics/observers/session_restore_foreground_tab_page_load_metrics_observer.h"
 #include "chrome/browser/page_load_metrics/observers/subresource_filter_metrics_observer.h"
 #include "chrome/browser/page_load_metrics/observers/tab_restore_page_load_metrics_observer.h"
 #include "chrome/browser/page_load_metrics/observers/ukm_page_load_metrics_observer.h"
@@ -129,6 +130,14 @@ void PageLoadMetricsEmbedder::RegisterObservers(
                 web_contents_);
     if (loading_predictor_observer)
       tracker->AddObserver(std::move(loading_predictor_observer));
+#if defined(OS_WIN) || defined(OS_MACOSX) || defined(OS_LINUX)

[chrisha, 2017/06/15 19:44:56]

OS_CHROMEOS as well?

[ducbui, 2017/06/15 23:27:46]

TabManager seems not supported on CHROMEOS.
BrowserProcessImpl::GetTabManager() returns non-null only if 
defined(OS_WIN) || defined(OS_MACOSX) || defined(OS_LINUX)
(https://cs.chromium.org/chromium/src/chrome/browser/browser_process_impl.cc?l...)

Does OS_LINUX include OS_CHROMEOS?

[chrisha, 2017/06/16 16:52:35]

It is supported on ChromeOS, so yes I suppose OS_LINUX must subsume OS_CHROMEOS.

I dislike the fragility of replicating the #ifdef logic here as well. Can we
make the code compile on all platforms and make CreateIfNeeded return null when
not necessary?

[ducbui, 2017/06/16 20:59:55]

That's a good point. I would remove the #if and make CreateIfNeeded() return
null when TabManager is not supported (i.e., BrowserProcess::GetTabManager()
returns null).

+    std::unique_ptr<page_load_metrics::PageLoadMetricsObserver>
+        session_restore_foreground_tab_observer =
+            SessionRestoreForegroundTabPageLoadMetricsObserver::
+                CreateIfNeeded();
+    if (session_restore_foreground_tab_observer)
+      tracker->AddObserver(std::move(session_restore_foreground_tab_observer));
+#endif
   } else {
     std::unique_ptr<page_load_metrics::PageLoadMetricsObserver>
         prerender_observer =