customtabs: Send time to first contentful paint to the caller.

chrome/android/java/src/org/chromium/chrome/browser/customtabs/CustomTabActivity.java

Index: chrome/android/java/src/org/chromium/chrome/browser/customtabs/CustomTabActivity.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/customtabs/CustomTabActivity.java b/chrome/android/java/src/org/chromium/chrome/browser/customtabs/CustomTabActivity.java
index d29d305fbe93421ee8de7a180eb26cd7ae8f3524..3a47d2d33f77136237c3c085f6341eef0f45c420 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/customtabs/CustomTabActivity.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/customtabs/CustomTabActivity.java
@@ -44,6 +44,7 @@ import org.chromium.chrome.browser.compositor.layouts.LayoutManagerDocument;
 import org.chromium.chrome.browser.datausage.DataUseTabUIManager;
 import org.chromium.chrome.browser.document.ChromeLauncherActivity;
 import org.chromium.chrome.browser.externalnav.ExternalNavigationDelegateImpl;
+import org.chromium.chrome.browser.metrics.PageLoadMetrics;
 import org.chromium.chrome.browser.net.spdyproxy.DataReductionProxySettings;
 import org.chromium.chrome.browser.pageinfo.WebsiteSettingsPopup;
 import org.chromium.chrome.browser.rappor.RapporServiceBridge;
@@ -97,15 +98,40 @@ public class CustomTabActivity extends ChromeActivity {
     private String mPrerenderedUrl;
     private boolean mHasPrerendered;
 
-    // Only the normal tab model is observed because there is no icognito tabmodel in Custom Tabs.
+    private static class PageLoadMetricsObserver implements PageLoadMetrics.Observer {
+        private final CustomTabsConnection mConnection;
+        private final CustomTabsSessionToken mSession;
+        private final WebContents mWebContents;
+
+        public PageLoadMetricsObserver(CustomTabsConnection connection,
+                CustomTabsSessionToken session, WebContents webContents) {
+            mConnection = connection;
+            mSession = session;
+            mWebContents = webContents;
+        }
+
+        @Override
+        public void onFirstContentfulPaint(WebContents webContents, long firstContentfulPaintMs) {
+            if (webContents != mWebContents) return;

[Bryan McQuade, 2016/07/26 14:29:33]

do we expect to hit this early return? in what cases might this happen?

[Benoit L, 2016/07/27 16:23:44]

Yes.

There can be multiple Custom Tabs active at the same time (each having its own
WebContents).

+
+            mConnection.notifyPageLoadMetric(

[Bryan McQuade, 2016/07/26 14:29:33]

we've found that it's less useful to track first contentful paints for pages
that get backgrounded. in particular, once backgrounded, the rendering pipeline
gets throttled, which means that even if a page reaches the point where it's
ready to paint, it won't actually paint until foregrounded again IIUC. so in
that case, time to first contentful paint ends up being time to re-enter
foreground.

do we have a clean way to track whether the page was backgrounded between nav
start and FCP? alternatively we can suppress notifications of FCP if the page
was backgrounded on the c++ side, which is straightforward.

[Benoit L, 2016/07/27 16:23:45]

Thanks for pointing that out!

There is a way to know that the page was backgrounded from the hosting
application, since Custom Tabs relay the information when a page visibility
changes.

+                    mSession, PageLoadMetrics.FIRST_CONTENTFUL_PAINT, firstContentfulPaintMs);
+        }
+    }
+
+    private PageLoadMetricsObserver mMetricsObserver;
+
+    // Only the normal tab model is observed because there is no incognito tabmodel in Custom Tabs.
     private TabModelObserver mTabModelObserver = new EmptyTabModelObserver() {
         @Override
         public void didAddTab(Tab tab, TabLaunchType type) {
+            PageLoadMetrics.addObserver(mMetricsObserver);
             tab.addObserver(mTabObserver);
         }
 
         @Override
         public void didCloseTab(int tabId, boolean incognito) {
+            PageLoadMetrics.removeObserver(mMetricsObserver);
             // Finish the activity after we intent out.
             if (getTabModelSelector().getCurrentModel().getCount() == 0) finish();
         }
@@ -393,6 +419,9 @@ public class CustomTabActivity extends ChromeActivity {
         tab.getTabRedirectHandler().updateIntent(getIntent());
         tab.getView().requestFocus();
         mTabObserver = new CustomTabObserver(getApplication(), mSession);
+
+        mMetricsObserver = new PageLoadMetricsObserver(
+                CustomTabsConnection.getInstance(getApplication()), mSession, webContents);
         tab.addObserver(mTabObserver);
         return tab;
     }