Without this `./mach run -b -- -M` would start fine but navigating to a site would fail with:
`thread 'ScriptThread PipelineId { namespace_id: PipelineNamespaceId(0), index: PipelineIndex(0) }' panicked at 'assertion failed: mozbrowser_enabled()', /Users/ulf/Documents/Code/servo/components/script/dom/htmliframeelement.rs:163`