The find_if lambdas in CustomElementRegistry and TraversableNavigable
were taking GC::Root parameters, causing implicit conversion from
GC::Ref (or copying of GC::Root) for each element visited. Each
GC::Root creation/destruction allocates and frees a RootImpl, making
linear scans over these vectors far more expensive than necessary.
Use auto const& to match the actual vector element types.
This was 2.3% of CPU time while opening a YouTube video.
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