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...instead of replacing if the property is already set.

This matches the spec[1] which was changed as a resolution from[2].

Intent thread: https://groups.google.com/a/chromium.org/d/msg/blink-dev/lzBoa1EAQpI/strfpNczAwAJ

[1]: https://drafts.csswg.org/cssom/#set-a-css-declaration
[2]: https://github.com/w3c/csswg-drafts/issues/1898

Bug: 782407
Change-Id: I4b72c0207a4cf734fc2e415bd62c7863103ec309
Reviewed-on: https://chromium-review.googlesource.com/1054867
Commit-Queue: Emilio Cobos Álvarez <emilio@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560180}