The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5f543cf03508ae5cc38f69a1871b99308859a2d8

commit 5f543cf03508ae5cc38f69a1871b99308859a2d8
Author: ericwilligers <ericwilligers@chromium.org>
Date: Wed Mar 22 06:54:13 2017

Web Animations: support cssOffset

When animating the CSS shorthand property 'offset', cssOffset is used
in Web Animations keyframes.

https://w3c.github.io/web-animations/#property-name-conversion

Note that when using CSS Animations or Web Animations on a shorthand property, and discrete animation applies to one of the longhand properties, that longhand property updates at 50% progress. When using CSS Transitions on a shorthand property, and discrete animation applies to one of the longhand properties, that longhand property updates immediately.

BUG= 700534 

Review-Url: https://codereview.chromium.org/2747523002
Cr-Commit-Position: refs/heads/master@{#458651}

[add] https://crrev.com/5f543cf03508ae5cc38f69a1871b99308859a2d8/third_party/WebKit/LayoutTests/animations/interpolation/border-interpolation.html
[add] https://crrev.com/5f543cf03508ae5cc38f69a1871b99308859a2d8/third_party/WebKit/LayoutTests/animations/interpolation/offset-interpolation.html
[modify] https://crrev.com/5f543cf03508ae5cc38f69a1871b99308859a2d8/third_party/WebKit/LayoutTests/animations/interpolation/resources/interpolation-test.js
[modify] https://crrev.com/5f543cf03508ae5cc38f69a1871b99308859a2d8/third_party/WebKit/Source/core/animation/AnimationInputHelpers.cpp