Bug 1486959 - Remove bogus assertion in ContentClient. r=nical

It's possible to trigger this assertion with asyncPaint = true and
bufferRotation != (0, 0). In this case though everything should work fine,
because we didn't start a new paint task, and we should be able to copy over
buffers. So we should be able to remove this assertion.

gfx/layers/client/ContentClient.cpp

@@ -195,7 +195,6 @@ ContentClient::BeginPaint(PaintedLayer* aLayer,
           newParameters.SetUnrotated();
           mBuffer->SetParameters(newParameters);
         } else {
-          MOZ_ASSERT(!result.mAsyncPaint);
           MOZ_ASSERT(GetFrontBuffer());
           mBuffer->Unlock();
           dest.mBufferRect = ComputeBufferRect(dest.mNeededRegion.GetBounds());