Fix crash on exit of Google Maps

QtWebKit may crash when exiting the new WebGL version of Google maps.

The problem is that under some circumstances the PlatformLayer is destroyed
before the GraphicsLayer that reference it, causing the GraphicsLayer to
access it during destruction.

Change-Id: Ia3d0f337dcb9b8b38e9e23f53bea669bd868f713
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>