Reland "Initial implementation of cc::ScrollTimeline"

This is a reland of 31974d24b5de701ff9c290b32e4dd543abec8146
Original change's description:
> Initial implementation of cc::ScrollTimeline
> 
> This creates a compositor side concept of a ScrollTimeline, and adds
> plumbing so that blink WorkletAnimations created with ScrollTimelines
> will create a cc::ScrollTimeline on the compositor side. This means
> that scroll-driven AnimationWorklets are now possible.
> 
> There are a number of caveats to this initial implementation:
>  * We never update the cc::ScrollTimeline, so if the scroller
>    changes on the blink side we won't know.
>  * We assume that the scroller is composited. If it isn't composited
>    during animation play() we will log a warning and fail to start,
>    but if it is composited at that time and later becomes non-composited
>    we will likely crash.
> 
> Bug: 776952
> 
> Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
> Change-Id: Id0342c62209b387130a69edca4ff3682748bd8a3
> Reviewed-on: https://chromium-review.googlesource.com/738437
> Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
> Reviewed-by: Robert Flack <flackr@chromium.org>
> Reviewed-by: dstockwell <dstockwell@chromium.org>
> Reviewed-by: Majid Valipour <majidvp@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#515645}

Bug: 776952
Change-Id: I372d8fd48c62a63b5c6b10688d3829e4ac2c3be1
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Reviewed-on: https://chromium-review.googlesource.com/764578
Reviewed-by: Robert Flack <flackr@chromium.org>
Reviewed-by: Eric Willigers <ericwilligers@chromium.org>
Reviewed-by: dstockwell <dstockwell@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515985}

• cc/BUILD.gn[diff]
• cc/animation/BUILD.gn[diff]
• cc/animation/DEPS[diff]
• cc/animation/animation_host.cc[diff]
• cc/animation/animation_host.h[diff]
• cc/animation/scroll_timeline.cc[Added - diff]
• cc/animation/scroll_timeline.h[Added - diff]
• cc/animation/scroll_timeline_unittest.cc[Added - diff]
• cc/animation/worklet_animation_player.cc[diff]
• cc/animation/worklet_animation_player.h[diff]
• cc/animation/worklet_animation_player_unittest.cc[diff]
• cc/test/animation_timelines_test_common.cc[diff]
• cc/test/layer_tree_test.cc[diff]
• cc/trees/layer_tree_host.cc[diff]
• cc/trees/layer_tree_host_impl.cc[diff]
• cc/trees/layer_tree_host_impl.h[diff]
• cc/trees/layer_tree_mutator.h[diff]
• cc/trees/mutator_host.h[diff]
• third_party/WebKit/LayoutTests/virtual/threaded/fast/animationworklet/animation-worklet-scroll-timeline-expected.html[Added - diff]
• third_party/WebKit/LayoutTests/virtual/threaded/fast/animationworklet/animation-worklet-scroll-timeline.html[Added - diff]
• third_party/WebKit/Source/core/animation/ScrollTimeline.h[diff]
• third_party/WebKit/Source/core/animation/WorkletAnimationBase.h[diff]
• third_party/WebKit/Source/core/animation/WorkletAnimationController.cpp[diff]
• third_party/WebKit/Source/core/animation/WorkletAnimationController.h[diff]
• third_party/WebKit/Source/core/dom/Document.cpp[diff]
• third_party/WebKit/Source/modules/animationworklet/AnimationWorkletGlobalScopeTest.cpp[diff]
• third_party/WebKit/Source/modules/animationworklet/Animator.cpp[diff]
• third_party/WebKit/Source/modules/animationworklet/Animator.h[diff]
• third_party/WebKit/Source/modules/animationworklet/WorkletAnimation.cpp[diff]
• third_party/WebKit/Source/modules/animationworklet/WorkletAnimation.h[diff]
• third_party/WebKit/Source/platform/animation/CompositorAnimationPlayer.cpp[diff]
• third_party/WebKit/Source/platform/animation/CompositorAnimationPlayer.h[diff]