Firefox exposes the DocumentTimeline constructor.

I've started looking at this. To make things easier I'm first doing some cleanup work to (effectively) shift the bulk of AnimationTimeline code to DocumentTimeline (see https://codereview.chromium.org/2944423003 and associated patches).

The AnimationTimeline/DocumentTimeline refactor has now landed and I've started working on the DocumentTimeline constructor.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7ee5b6192d46b7a25c3670c1aa8b10547e1057ef

commit 7ee5b6192d46b7a25c3670c1aa8b10547e1057ef
Author: suzyh <suzyh@chromium.org>
Date: Fri Jun 30 02:19:48 2017

Implement DocumentTimeline constructor

This patch adds a constructor to DocumentTimeline to match the spec at
http://w3c.github.io/web-animations/#the-documenttimeline-interface. The
constructor enables the zero time of the timeline to be set at
something other than the navigationStart moment.

Changing the playback rate of the timeline (a functionality provided for
DevTools support) still modifies this zero time. The initial value for
the zero time is saved in the origin_time_ member.

BUG= 674514 , 624639 , 600248 

Review-Url: https://codereview.chromium.org/2960733004
Cr-Commit-Position: refs/heads/master@{#483594}

[delete] https://crrev.com/f2703bb4a656cdd8b1c058040fac0eac101643c0/third_party/WebKit/LayoutTests/external/wpt/web-animations/interfaces/DocumentTimeline/constructor-expected.txt
[modify] https://crrev.com/7ee5b6192d46b7a25c3670c1aa8b10547e1057ef/third_party/WebKit/Source/core/animation/DocumentTimeline.cpp
[modify] https://crrev.com/7ee5b6192d46b7a25c3670c1aa8b10547e1057ef/third_party/WebKit/Source/core/animation/DocumentTimeline.h
[modify] https://crrev.com/7ee5b6192d46b7a25c3670c1aa8b10547e1057ef/third_party/WebKit/Source/core/animation/DocumentTimeline.idl
[add] https://crrev.com/7ee5b6192d46b7a25c3670c1aa8b10547e1057ef/third_party/WebKit/Source/core/animation/DocumentTimelineOptions.idl
[modify] https://crrev.com/7ee5b6192d46b7a25c3670c1aa8b10547e1057ef/third_party/WebKit/Source/core/animation/DocumentTimelineTest.cpp
[modify] https://crrev.com/7ee5b6192d46b7a25c3670c1aa8b10547e1057ef/third_party/WebKit/Source/core/core_idl_files.gni