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The baseline-position syntax has changed recently, so we need to update
the CSS properties using the old syntax. This patch address only the
content-alignment (align-content and justify-content).

I used this patch to adapt the implementation of the parsing logic for
these properties to the new Blink's CSS Parsing Design.

The new baseline syntax is "[first | last ]? baseline" which implies
modifying the parsing and computed value logic.

There are several layout tests affected by this change, so I'll update
them accordingly.

BUG=565883

Review-Url: https://codereview.chromium.org/2750843002
Cr-Commit-Position: refs/heads/master@{#457434}