[CustomTabs] Report the navigation start as absolute time

chrome/browser/page_load_metrics/observers/android_page_load_metrics_observer.cc

 // Copyright 2016 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/page_load_metrics/observers/android_page_load_metrics_observer.h"
 
 #include <string>
 
 #include "base/android/jni_android.h"
 #include "base/android/jni_string.h"
 #include "base/android/scoped_java_ref.h"
+#include "base/time/time.h"
 #include "chrome/browser/page_load_metrics/page_load_metrics_util.h"
 #include "content/public/browser/browser_thread.h"
 #include "content/public/browser/web_contents.h"
 #include "jni/PageLoadMetrics_jni.h"
 #include "url/gurl.h"
 
 AndroidPageLoadMetricsObserver::AndroidPageLoadMetricsObserver(
     content::WebContents* web_contents)
     : web_contents_(web_contents) {}
 
+page_load_metrics::PageLoadMetricsObserver::ObservePolicy
+AndroidPageLoadMetricsObserver::OnStart(
+    content::NavigationHandle* navigation_handle,
+    const GURL& currently_committed_url,
+    bool started_in_foreground) {
+  if (!navigation_handle)
+    return STOP_OBSERVING;
+  navigation_start_ = navigation_handle->NavigationStart();
+  return CONTINUE_OBSERVING;
+}
+
 void AndroidPageLoadMetricsObserver::OnFirstContentfulPaint(
     const page_load_metrics::PageLoadTiming& timing,
     const page_load_metrics::PageLoadExtraInfo& extra_info) {
   DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
   int64_t first_contentful_paint_ms =
       timing.first_contentful_paint->InMilliseconds();
   base::android::ScopedJavaLocalRef<jobject> java_web_contents =
       web_contents_->GetJavaWebContents();
   JNIEnv* env = base::android::AttachCurrentThread();
   Java_PageLoadMetrics_onFirstContentfulPaint(
-      env, java_web_contents, static_cast<jlong>(first_contentful_paint_ms));
+      env, java_web_contents,
+      static_cast<jlong>(navigation_start_.ToInternalValue() / 1000),

[Bernhard Bauer, 2016/11/15 16:12:02]

From
https://cs.chromium.org/chromium/src/base/time/time.h?sq=package:chromium&dr=...:
"Please don't use this and do arithmetic on it, as it is more error prone than
using the provided operators."

Instead, you could do what Blink does:

  (navigation_start_ - base::TimeTicks()).InMilliseconds()

+      static_cast<jlong>(first_contentful_paint_ms));
 }