Re: [w3c/webcomponents] Shadow tree encapsulation and Element Timing (#816)

For the purpose of element timing, elements with shadow trees should be treated like any builtin element with shadow trees. Given element timing currently only supports measuring `img` and SVG `image` elements and elements with background images for now, the only shadow host with a background image is measurable as is.

Since elements inside shadow trees are considered as implementation details, no element marked with `elementtiming` inside shadow trees should be reported to the performance timeline.

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