Reflow comments in core/css

Note that some ASSERTs became DCHECKs, and some bitfields became unsigned
instead of uint16_t, because I reflowed a comment next to them.

BUG=563793

Review-Url: https://codereview.chromium.org/2392343005
Cr-Commit-Position: refs/heads/master@{#423514}
92 files changed