Fix broken AddToHomescreenManagerTest#testAddAdaptableShortcut

Previously we had AddToHomescreenManagerTest#testAddAdaptableShortcut
broken because we were not mocking the AddToHomescreenView fully.
This CL fixes this.

Bug: 977173
Change-Id: I643d4e26c354ae14792a2e85f720edad712a60cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1832287Reviewed-by: Peter Kotwicz <pkotwicz@chromium.org>
Commit-Queue: Peter Kotwicz <pkotwicz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#701282}

chrome/android/javatests/src/org/chromium/chrome/browser/webapps/addtohomescreen/AddToHomescreenManagerTest.java

@@ -166,6 +166,11 @@ public class AddToHomescreenManagerTest {
                 public void onIconAvailable(Bitmap icon) {
                     TestAddToHomescreenManager.this.addToHomescreen(mTitle);
                 }
+
+                @Override
+                public void onAdaptableIconAvailable(Bitmap icon) {
+                    TestAddToHomescreenManager.this.addToHomescreen(mTitle);
+                }
             };
         }
     }