Adding JUnit tests for MediaNotificationManager

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

Index: chrome/android/java/src/org/chromium/chrome/browser/AppHooks.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/AppHooks.java b/chrome/android/java/src/org/chromium/chrome/browser/AppHooks.java
index a4f3297c19d1cdb34e9a6f8b095d51a420ee90e9..6740fdda86b06ef6e399dc93c7d3d34067ffd42d 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/AppHooks.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/AppHooks.java
@@ -10,6 +10,7 @@ import android.os.Handler;
 import android.os.Looper;
 
 import org.chromium.base.ContextUtils;
+import org.chromium.base.VisibleForTesting;
 import org.chromium.base.annotations.CalledByNative;
 import org.chromium.chrome.browser.banners.AppDetailsDelegate;
 import org.chromium.chrome.browser.customtabs.CustomTabsConnection;
@@ -52,6 +53,14 @@ import org.chromium.policy.CombinedPolicyProvider;
 public abstract class AppHooks {
     private static AppHooksImpl sInstance;
 
+    /**
+     * Sets a mocked instance for testing.
+     */
+    @VisibleForTesting
+    public static void setInstanceForTesting(AppHooksImpl instance) {
+        sInstance = instance;
+    }
+
     @CalledByNative
     public static AppHooks get() {
         if (sInstance == null) {