Bug 1587695 - Fix redundant SwapChain re-creation r=nical

Since Bug 1570879, SwapChain is created with alpha at first, then the SwapChain is typically re-created at first RenderCompositorANGLE::BeginFrame() calle, since non Glass window is common since Windows 10. The re-creation is redundant.

Differential Revision: https://phabricator.services.mozilla.com/D48800

--HG--
extra : moz-landing-system : lando

gfx/webrender_bindings/RenderCompositorANGLE.cpp

@@ -273,7 +273,8 @@ void RenderCompositorANGLE::CreateSwapChainForDCompIfPossible(
   MOZ_ASSERT(XRE_IsGPUProcess());
 
   bool useTripleBuffering = gfx::gfxVars::UseWebRenderTripleBufferingWin();
-  bool useAlpha = true;
+  // Non Glass window is common since Windows 10.
+  bool useAlpha = false;
   RefPtr<IDXGISwapChain1> swapChain1 =
       CreateSwapChainForDComp(useTripleBuffering, useAlpha);
   if (swapChain1) {