Previously, the show more button was not ever displayed until all
related changes sections had finished loading.
Because it's possible for any section loaded to start overflowing
content, it should be checked after each one completes.
A new event gets fired from gr-related-changes view after each promise
resolves. The change view can then determine if the content is
overflowing at that time and add the show more button. In the event
that the content overflows after only some sections has loaded, it will
appear sooner than before.
Bug: Issue 7106
Change-Id: I87006e0780e362fe6ef1a59a35bbc08d903ec4e7
9afd569022