android: Add a method to let Java know whether a prerender has finished loading.

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

Index: chrome/android/java/src/org/chromium/chrome/browser/customtabs/CustomTabsConnection.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/customtabs/CustomTabsConnection.java b/chrome/android/java/src/org/chromium/chrome/browser/customtabs/CustomTabsConnection.java
index 0eed1965b1bb7ebe54c9dad6e23278adcca9351a..7cac67779ee8228c79b825dfe48e03522bf814fd 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/customtabs/CustomTabsConnection.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/customtabs/CustomTabsConnection.java
@@ -77,7 +77,8 @@ public class CustomTabsConnection extends ICustomTabsService.Stub {
     private static AtomicReference<CustomTabsConnection> sInstance =
             new AtomicReference<CustomTabsConnection>();
 
-    private static final class PrerenderedUrlParams {
+    @VisibleForTesting
+    static final class PrerenderedUrlParams {
         public final IBinder mSession;
         public final WebContents mWebContents;
         public final String mUrl;
@@ -94,12 +95,13 @@ public class CustomTabsConnection extends ICustomTabsService.Stub {
         }
     }
 
+    @VisibleForTesting
+    PrerenderedUrlParams mPrerender;
     protected final Application mApplication;
     private final boolean mLogRequests;
     private final AtomicBoolean mWarmupHasBeenCalled = new AtomicBoolean();
     private final ClientManager mClientManager;
     private ExternalPrerenderHandler mExternalPrerenderHandler;
-    private PrerenderedUrlParams mPrerender;
     private WebContents mSpareWebContents;
 
     /**