[WebLayer] Make register/unregister tab callback take NonNull callback

This callback should never be null.

Bug: 1062962
Change-Id: Ie81c2f52914823df3fa2e55a0d2633779d18a826
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2305117
Auto-Submit: Clark DuVall <cduvall@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789692}

weblayer/public/java/org/chromium/weblayer/Tab.java

@@ -282,12 +282,12 @@ public class Tab {
         return mMediaCaptureController;
     }
 
-    public void registerTabCallback(@Nullable TabCallback callback) {
+    public void registerTabCallback(@NonNull TabCallback callback) {
         ThreadCheck.ensureOnUiThread();
         mCallbacks.addObserver(callback);
     }
 
-    public void unregisterTabCallback(@Nullable TabCallback callback) {
+    public void unregisterTabCallback(@NonNull TabCallback callback) {
         ThreadCheck.ensureOnUiThread();
         mCallbacks.removeObserver(callback);
     }