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}
bool RenderWidgetHostViewAura::IsMouseLocked() {
  return event_handler_->mouse_locked();
}

bool RenderWidgetHostViewAura::IsMouseLocked() {
  return event_handler_->mouse_locked();
}
