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}
RenderWidgetHostViewAura::AccessibilityGetAcceleratedWidget() {
#if defined(OS_WIN)
  if (legacy_render_widget_host_HWND_)
    return legacy_render_widget_host_HWND_->hwnd();
#endif
  return gfx::kNullAcceleratedWidget;
}

RenderWidgetHostViewAura::AccessibilityGetAcceleratedWidget() {
#if defined(OS_WIN)
  if (legacy_render_widget_host_HWND_)
    return legacy_render_widget_host_HWND_->hwnd();
#endif
  return gfx::kNullAcceleratedWidget;
}
