Firefox exposes the Animation constructor (but there doesn't appear to be any way of calling it with the correct data types since KeyframeEffectReadonly isn't exposed yet).

I have a WIP patch https://codereview.chromium.org/2806623003. Implementing requires a web-visible change even without shipping the functional constructor; I have sent a PSA to blink-dev about this: https://groups.google.com/a/chromium.org/d/topic/blink-dev/WkNcwh6aISA/discussion

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8427660616e176502578ec0a82982b3ed3038864

commit 8427660616e176502578ec0a82982b3ed3038864
Author: suzyh <suzyh@chromium.org>
Date: Fri Jun 09 06:24:16 2017

Implement Animation constructor

This patch changes the Animation object from NoInterfaceObject to having
a constructor, and implements two corresponding Animation::create
functions. The implementations are only exposed behind the
--enable-experimental-web-platform-features flag in the Window context.

Test expectations are updated accordingly.

BUG= 674513 

Review-Url: https://codereview.chromium.org/2806623003
Cr-Commit-Position: refs/heads/master@{#478215}

[modify] https://crrev.com/8427660616e176502578ec0a82982b3ed3038864/third_party/WebKit/LayoutTests/TestExpectations
[modify] https://crrev.com/8427660616e176502578ec0a82982b3ed3038864/third_party/WebKit/LayoutTests/external/wpt/web-animations/interfaces/Animatable/animate-expected.txt
[modify] https://crrev.com/8427660616e176502578ec0a82982b3ed3038864/third_party/WebKit/LayoutTests/external/wpt/web-animations/interfaces/Animation/constructor-expected.txt
[delete] https://crrev.com/01f09fba7a7f4510b216ecb16c55ff52d2a2ba33/third_party/WebKit/LayoutTests/external/wpt/web-animations/interfaces/Animation/effect-expected.txt
[delete] https://crrev.com/01f09fba7a7f4510b216ecb16c55ff52d2a2ba33/third_party/WebKit/LayoutTests/external/wpt/web-animations/interfaces/Animation/finish-expected.txt
[modify] https://crrev.com/8427660616e176502578ec0a82982b3ed3038864/third_party/WebKit/LayoutTests/external/wpt/web-animations/interfaces/Animation/idlharness-expected.txt
[delete] https://crrev.com/01f09fba7a7f4510b216ecb16c55ff52d2a2ba33/third_party/WebKit/LayoutTests/external/wpt/web-animations/interfaces/Animation/startTime-expected.txt
[modify] https://crrev.com/8427660616e176502578ec0a82982b3ed3038864/third_party/WebKit/LayoutTests/external/wpt/web-animations/interfaces/Document/getAnimations-expected.txt
[modify] https://crrev.com/8427660616e176502578ec0a82982b3ed3038864/third_party/WebKit/LayoutTests/external/wpt/web-animations/interfaces/KeyframeEffect/setTarget-expected.txt
[delete] https://crrev.com/01f09fba7a7f4510b216ecb16c55ff52d2a2ba33/third_party/WebKit/LayoutTests/external/wpt/web-animations/timing-model/animations/current-time-expected.txt
[modify] https://crrev.com/8427660616e176502578ec0a82982b3ed3038864/third_party/WebKit/LayoutTests/external/wpt/web-animations/timing-model/animations/reversing-an-animation-expected.txt
[delete] https://crrev.com/01f09fba7a7f4510b216ecb16c55ff52d2a2ba33/third_party/WebKit/LayoutTests/external/wpt/web-animations/timing-model/animations/set-the-animation-start-time-expected.txt
[modify] https://crrev.com/8427660616e176502578ec0a82982b3ed3038864/third_party/WebKit/LayoutTests/external/wpt/web-animations/timing-model/animations/set-the-target-effect-of-an-animation-expected.txt
[modify] https://crrev.com/8427660616e176502578ec0a82982b3ed3038864/third_party/WebKit/LayoutTests/external/wpt/web-animations/timing-model/animations/set-the-timeline-of-an-animation-expected.txt
[modify] https://crrev.com/8427660616e176502578ec0a82982b3ed3038864/third_party/WebKit/LayoutTests/virtual/service-worker-navigation-preload-disabled/webexposed/global-interface-listing-expected.txt
[modify] https://crrev.com/8427660616e176502578ec0a82982b3ed3038864/third_party/WebKit/LayoutTests/webexposed/global-interface-listing-expected.txt
[modify] https://crrev.com/8427660616e176502578ec0a82982b3ed3038864/third_party/WebKit/LayoutTests/webexposed/web-animations-api-expected.txt
[modify] https://crrev.com/8427660616e176502578ec0a82982b3ed3038864/third_party/WebKit/Source/core/animation/Animation.cpp
[modify] https://crrev.com/8427660616e176502578ec0a82982b3ed3038864/third_party/WebKit/Source/core/animation/Animation.h
[modify] https://crrev.com/8427660616e176502578ec0a82982b3ed3038864/third_party/WebKit/Source/core/animation/Animation.idl

In the end, this did not result in a web-visible change: the constructor is exposed only behind the WebAnimationsAPI flag. Since this bug was about implementing the constructor, rather than shipping it, marking as fixed.