Allocate a FrameSinkId for RenderWidgetHostViewAura in mus+ash

RenderWidgetHostViewChildFrame expects its parent to have a valid
FrameSinkId. Make sure RenderWidgetHostViewAura has a FrameSinkId even
if DelegatedFrameHost is not used (in mus+ash).

BUG=706553
TBR=jam@chromium.org

Review-Url: https://codereview.chromium.org/2847253003
Cr-Commit-Position: refs/heads/master@{#468179}
  void RemoveAncestorObservers() {
    for (auto* ancestor : ancestors_)
      ancestor->RemoveObserver(this);
    ancestors_.clear();
  }

  void RemoveAncestorObservers() {
    for (auto* ancestor : ancestors_)
      ancestor->RemoveObserver(this);
    ancestors_.clear();
  }
