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The CSS Transitions specification recently added a section specifying how timing
functions should be serialized:
https://hg.csswg.org/drafts/rev/12d32649a91f
https://drafts.csswg.org/css-transitions/#serializing-a-timing-function

This patch changes how step timing functions are serialized in order to bring it
in line with the spec. The corresponding parsing function devtools/front_end is
also updated.

The removal of "step-middle" and "steps(x, middle)" as valid values is deferred
until frames() is implemented (see crbug.com/646265).

BUG=631875,600248

Review-Url: https://codereview.chromium.org/2330283004
Cr-Commit-Position: refs/heads/master@{#418704}