Rename MostVisitedSites.MostVisitedURLsObserver to Observer.

BUG=677672

Review-Url: https://codereview.chromium.org/2697543002
Cr-Commit-Position: refs/heads/master@{#449958}
MostVisitedSitesBridge::MostVisitedSitesBridge(Profile* profile)
    : most_visited_(ChromeMostVisitedSitesFactory::NewForProfile(profile)) {
  content::URLDataSource::Add(profile, new ThumbnailListSource(profile));
  DCHECK(!profile->IsOffTheRecord());
}

MostVisitedSitesBridge::MostVisitedSitesBridge(Profile* profile)
    : most_visited_(ChromeMostVisitedSitesFactory::NewForProfile(profile)) {
  content::URLDataSource::Add(profile, new ThumbnailListSource(profile));
  DCHECK(!profile->IsOffTheRecord());
}
