Enable spare RenderProcessHost to be preinitialized.

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

Index: chrome/android/java/src/org/chromium/chrome/browser/WarmupManager.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/WarmupManager.java b/chrome/android/java/src/org/chromium/chrome/browser/WarmupManager.java
index 7c6ccb38bc737891664f5ace3b09ebee4922aeba..8fff2fc1a8a03402872b0a359cdd6865da8a04fa 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/WarmupManager.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/WarmupManager.java
@@ -248,6 +248,22 @@ public final class WarmupManager {
         }
     }
 
+    /**
+     * 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.
      *
@@ -314,4 +330,5 @@ public final class WarmupManager {
     }
 
     private static native void nativePreconnectUrlAndSubresources(Profile profile, String url);
+    private static native void nativeWarmupSpareRenderer(Profile profile);
 }