Add back extra_application_definitions block to AndroidManifest.xml

The extra_application_definitions is still used by downstream manifests
even if not used upstream.

Change-Id: I610bb6c50df2ed828c503b4567c1bc0bfc3a8f63
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2517624Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Mohamed Heikal <mheikal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823590}

chrome/android/java/AndroidManifest.xml

@@ -1166,6 +1166,8 @@ by a child template that "extends" this file.
         </activity>
         {% endif %}
 
+        {% block extra_application_definitions %}
+        {% endblock %}
       {% endmacro %}
       {% if not definitions_in_split %}
       {{ application_definitions() }}