Stylo: match both ::placehoder & ::moz-placeholder for placeholder ps…
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The first patch of "Bug 1348490 - stylo: need support for ::-moz-placeholder pseudo element"
Part 2 need to be landed immediately after Part 1 be merged into servo repo:
https://reviewboard.mozilla.org/r/141264/diff/9#index_header
Bugzilla link:
https://bugzilla.mozilla.org/show_bug.cgi?id=1348490
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