Enable spare RenderProcessHost to be preinitialized.

chrome/android/java/src/org/chromium/chrome/browser/WarmupManager.java

 // Copyright 2015 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.
 
 package org.chromium.chrome.browser;
 
 import android.annotation.SuppressLint;
 import android.content.Context;
 import android.os.AsyncTask;
 import android.os.StrictMode;
@@ skipping 231 matching lines @@
                 // Note that if two requests come for the same URL with two
                 // different profiles, the last one will win.
                 mPendingPreconnectWithProfile.put(url, profile);
             } else {
                 nativePreconnectUrlAndSubresources(profile, url);
             }
         }
     }
 
     /**
+     * Warms up a spare, empty renderer that may be used for subsequent navigations.

[Benoit L, 2017/06/15 22:17:11]

Can this be named "RenderProcessHost"?

That would hopefully lessen confusion between this and the other one.
Also, could you discard the spare WebContents (if any) when this is called? To
avoid avoid several empty renderers.

[mattcary, 2017/06/16 08:18:56]

Done.

+     *
+     * The spare renderer will be used automatically in subsequent navigations. There is
+     * nothing further the WarmupManager needs to do to enable that use.
+     *
+     * This uses a different mechanism than createSpareWebContents, below, and is subject
+     * to fewer restrictions.
+     *
+     * This must be called from the UI thread.
+     */
+    public void createSpareRenderer(Profile profile) {
+        ThreadUtils.assertOnUiThread();
+        nativeWarmupSpareRenderer(profile);
+    }
+
+    /**
      * Creates and initializes a spare WebContents, to be used in a subsequent navigation.
      *
      * This creates a renderer that is suitable for any navigation. It can be picked up by any tab.
      * Can be called multiple times, and must be called from the UI thread.
      * Note that this is a no-op on low-end devices.
      */
     public void createSpareWebContents() {
         ThreadUtils.assertOnUiThread();
         if (mSpareWebContents != null || SysUtils.isLowEndDevice()) return;
         mSpareWebContents = WebContentsFactory.createWebContentsWithWarmRenderer(false, false);
@@ skipping 45 matching lines @@
         mSpareWebContents = null;
         mObserver = null;
     }
 
     private static void recordWebContentsStatus(int status) {
         RecordHistogram.recordEnumeratedHistogram(
                 WEBCONTENTS_STATUS_HISTOGRAM, status, WEBCONTENTS_STATUS_COUNT);
     }
 
     private static native void nativePreconnectUrlAndSubresources(Profile profile, String url);
+    private static native void nativeWarmupSpareRenderer(Profile profile);
 }