Bug 876129 - Part 1: Continue walking up the frame tree when hitting a transparent frame as we're trying to determine whether a given element's background is dark or not; r=roc a=lsblakk

This heuristic improvement gives us correct dark background detection in
pdf.js, among other cases.

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extra : transplant_source : %D9%21i%2B%E8%8F%B0%F5%87k%21iV%C7%C8%14%91sG%F1
Bug 876129 - Part 2: Prefer to look at <body> instead of <html> when determining whether the background color of a page is dark or not; r=roc a=lsblakk

This seems to be a slightly better heuristic to follow the CSS Rendering code
in all cases, since the background color for the body element often overrides
that of the html element.

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extra : transplant_source : %15%9Bp%E1tj5%A8%E8%ED%C1%D2%7FpuW%5Dj%BF%CA