When high contrast mode is enabled, title bar is drawn as transparent and on-client area rendering by DWM is shown. But when compositor window in GPU process is used, the on-client area rendering was not shown. To address the proboem, window needs to be cleard as transparent and SwapChain of compositor window needs to be DXGI_ALPHA_MODE_PREMULTIPLIED.

WinCompositorWidget::mTransparencyMode is changed to atomic, since it is accessed from compositor thread and render thread.

Differential Revision: https://phabricator.services.mozilla.com/D48302

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extra : moz-landing-system : lando