Only show context menu on notifications that have a settings button.

Thus system notifications, which can't be disabled anyway, stop showing
a context menu that is at best irrelevant and at worst broken.

TBR: sky@chromium.org,rsesek@chromium.org
Bug: 769353,769355
Change-Id: Ic0310be63f4f245b0a2f2cd2ccd3bc06b116b717
Reviewed-on: https://chromium-review.googlesource.com/702641
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#507453}

• ash/message_center/message_center_view.cc[diff]
• ash/system/web_notification/web_notification_tray.cc[diff]
• ash/system/web_notification/web_notification_tray.h[diff]
• chrome/browser/ui/cocoa/notifications/message_center_tray_bridge.h[diff]
• chrome/browser/ui/cocoa/notifications/message_center_tray_bridge.mm[diff]
• chrome/browser/ui/views/message_center/web_notification_tray.cc[diff]
• chrome/browser/ui/views/message_center/web_notification_tray.h[diff]
• ui/message_center/fake_message_center_tray_delegate.cc[diff]
• ui/message_center/fake_message_center_tray_delegate.h[diff]
• ui/message_center/message_center_tray.cc[diff]
• ui/message_center/message_center_tray_delegate.h[diff]
• ui/message_center/message_center_tray_unittest.cc[diff]
• ui/message_center/notification_delegate.h[diff]
• ui/message_center/views/message_popup_collection.cc[diff]