Bug 1779410 - Ensure SwapChainPresenter destructor is called before we access the front buffer. r=aosmond

Differential Revision: https://phabricator.services.mozilla.com/D151732

dom/canvas/WebGLContext.cpp

@@ -1058,6 +1058,7 @@ void WebGLContext::CopyToSwapChain(WebGLFramebuffer* const srcFb,
     return;
   }
 
+  {
     // ColorSpace will need to be part of SwapChainOptions for DTWebgl.
     const auto colorSpace = ToColorSpace2(mOptions);
     auto presenter = srcFb->mSwapChain.Acquire(size, colorSpace);
@@ -1067,7 +1068,6 @@ void WebGLContext::CopyToSwapChain(WebGLFramebuffer* const srcFb,
       return;
     }
 
-  {
     const ScopedFBRebinder saveFB(this);
 
     const auto destFb = presenter->Fb();