On cc side we use an optional for local time and it has the expected behavior.
On worklet side, unset local time is treated as zero time [1]. We should fix this and also allow undefined as a valid value to be explicitly used as well.

[1] https://cs.chromium.org/chromium/src/third_party/blink/renderer/modules/animationworklet/effect_proxy.h?sq=package:chromium&dr=CSs&g=0&l=37

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/82b3d45811c2ce34b664eacd55258a2e0d367c12

commit 82b3d45811c2ce34b664eacd55258a2e0d367c12
Author: Majid Valipour <majidvp@chromium.org>
Date: Fri Aug 24 15:30:01 2018

[animation-worklet] Support undefined as default localTime value

EffectProxy.localTime should accept undefined as its local time and treat
that as meaning that the keyframe effect is not in effect.


Note:
This is a partial fix. We still need to make sure that any applied effect
is cleared once localTime is set to undefined. This patch only ensures that
when the initial value is not set, or set to undefined the keyframe effect
is not applied.

Test: virtual/threaded/fast/animationworklet/worklet-animation-local-time-undefined.html

Bug: 870711
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I2fdc716c331eb11a9b3eed524ac703f50a222a49
Reviewed-on: https://chromium-review.googlesource.com/1181935
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Reviewed-by: Yi Gu <yigu@chromium.org>
Commit-Queue: Majid Valipour <majidvp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585849}
[modify] https://crrev.com/82b3d45811c2ce34b664eacd55258a2e0d367c12/cc/animation/animation_host_unittest.cc
[modify] https://crrev.com/82b3d45811c2ce34b664eacd55258a2e0d367c12/cc/trees/layer_tree_mutator.cc
[modify] https://crrev.com/82b3d45811c2ce34b664eacd55258a2e0d367c12/cc/trees/layer_tree_mutator.h
[add] https://crrev.com/82b3d45811c2ce34b664eacd55258a2e0d367c12/third_party/WebKit/LayoutTests/virtual/threaded/fast/animationworklet/worklet-animation-local-time-undefined-expected.html
[add] https://crrev.com/82b3d45811c2ce34b664eacd55258a2e0d367c12/third_party/WebKit/LayoutTests/virtual/threaded/fast/animationworklet/worklet-animation-local-time-undefined.html
[modify] https://crrev.com/82b3d45811c2ce34b664eacd55258a2e0d367c12/third_party/blink/renderer/modules/animationworklet/BUILD.gn
[modify] https://crrev.com/82b3d45811c2ce34b664eacd55258a2e0d367c12/third_party/blink/renderer/modules/animationworklet/animation_worklet_global_scope.cc
[modify] https://crrev.com/82b3d45811c2ce34b664eacd55258a2e0d367c12/third_party/blink/renderer/modules/animationworklet/animator.cc
[add] https://crrev.com/82b3d45811c2ce34b664eacd55258a2e0d367c12/third_party/blink/renderer/modules/animationworklet/effect_proxy.cc
[modify] https://crrev.com/82b3d45811c2ce34b664eacd55258a2e0d367c12/third_party/blink/renderer/modules/animationworklet/effect_proxy.h
[modify] https://crrev.com/82b3d45811c2ce34b664eacd55258a2e0d367c12/third_party/blink/renderer/modules/animationworklet/effect_proxy.idl