[Android] Add @VisibleForTesting to getMobileFolderId().

We no longer use that function.

TBR=tedchoc, ianwen
BUG=424244

Review URL: https://codereview.chromium.org/674593002

Cr-Commit-Position: refs/heads/master@{#300737}

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

@@ -268,6 +268,7 @@ public class BookmarksBridge {
     /**
      * @return The BookmarkId for Mobile folder node
      */
+    @VisibleForTesting
     public BookmarkId getMobileFolderId() {
         assert mIsNativeBookmarkModelLoaded;
         return nativeGetMobileFolderId(mNativeBookmarksBridge);