Bug 1089364. Avoid trying to resize the swap chain to a negative size. r=bas

This was causing GetBuffer to return DXGI_ERROR_INVALID_CALL

--HG--
extra : rebase_source : 49e62fd5dda1e28268218cffca4ea1ce7af1b6ea

gfx/layers/d3d11/CompositorD3D11.cpp

@@ -935,7 +935,7 @@ CompositorD3D11::VerifyBufferSize()
 
   if ((swapDesc.BufferDesc.Width == mSize.width &&
        swapDesc.BufferDesc.Height == mSize.height) ||
-      mSize.width == 0 || mSize.height == 0) {
+      mSize.width <= 0 || mSize.height <= 0) {
     return;
   }