customtabs: Pre-create a renderer in mayLaunchUrl().

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

Index: chrome/android/java/src/org/chromium/chrome/browser/customtabs/CustomTab.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/customtabs/CustomTab.java b/chrome/android/java/src/org/chromium/chrome/browser/customtabs/CustomTab.java
index 53312f14b1332038f4fc0bcbc76dc6f4cfca5aea..e970e1c788b734c2af7c19d3ea567d0e02b2bc64 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/customtabs/CustomTab.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/customtabs/CustomTab.java
@@ -152,6 +152,7 @@ public class CustomTab extends ChromeTab {
 
     private CustomTabObserver mTabObserver;
     private final boolean mEnableUrlBarHiding;
+    private boolean mShouldReplaceCurrentEntry;
 
     /**
      * Construct an CustomTab. Note that url and referrer given here is only used to retrieve a
@@ -167,6 +168,11 @@ public class CustomTab extends ChromeTab {
                 CustomTabsConnection.getInstance(activity.getApplication());
         WebContents webContents = customTabsConnection.takePrerenderedUrl(session, url, referrer);
         if (webContents == null) {
+            webContents = customTabsConnection.takeSpareWebContents();
+            // TODO(lizeb): Remove this once crbug.com/521729 is fixed.
+            if (webContents != null) mShouldReplaceCurrentEntry = true;
+        }
+        if (webContents == null) {
             webContents = WebContentsFactory.createWebContents(isIncognito(), false);
         }
         initialize(webContents, activity.getTabContentManager(), false);
@@ -184,6 +190,8 @@ public class CustomTab extends ChromeTab {
      */
     void loadUrlAndTrackFromTimestamp(LoadUrlParams params, long timestamp) {
         mTabObserver.trackNextPageLoadFromTimestamp(timestamp);
+        if (mShouldReplaceCurrentEntry) params.setShouldReplaceCurrentEntry(true);
+        mShouldReplaceCurrentEntry = false;
         loadUrl(params);
     }