[AccCanvas] Wait on GLFence only if supported

Author: magomez

Source/WebCore/platform/graphics/skia/ImageBufferSkiaAcceleratedBackend.cpp

@@ -259,7 +259,8 @@ void ImageBufferSkiaAcceleratedBackend::swapBuffersIfNeeded()
     IntSize textureSize(info.width(), info.height());
     if (!m_texture.back)
         m_texture.back = BitmapTexture::create(textureSize, BitmapTexture::Flags::SupportsAlpha);
-    fence->serverWait();
+    if (fence)
+        fence->serverWait();
     m_texture.back->copyFromExternalTexture(textureInfo.fID);
     fence = GLFence::create();
     std::swap(m_texture.back, m_texture.front);