Revert "Remove tracking of which animating elements exist."

This reverts commit 98160c6cfdbde6810dea0f7381900875ac45974d.

There are still underlying issues to be worked out.

Original change's description:
> Remove tracking of which animating elements exist.
>
> Prior to property trees, composited animations were created before their
> corresponding layers existed. However, the blink lifecycle now ensures
> that composited animations are created in sync with their property tree
> nodes such that they always exist in the tree. Removing this code saves
> time and will unlock further simplifications of the code.
>
> The ui animation infrastructure eagerly creates layers before their
> animations so it doesn't have this issue and the LayerAnimator class
> ensures that animations are cleaned up on Layer removal.
>
> Bug: 900241
> Change-Id: If854cf67ba8bd21e727e42475d26ce1a846a7674
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3200347
> Reviewed-by: David Baron <[email protected]>
> Reviewed-by: Philip Rogers <[email protected]>
> Commit-Queue: Robert Flack <[email protected]>
> Cr-Commit-Position: refs/heads/main@{#975833}

Bug: 900241
Change-Id: I8aeae7d6d0b3147362b9401c3c4bbed0df397617
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3497860
Bot-Commit: Rubber Stamper <[email protected]>
Commit-Queue: Robert Flack <[email protected]>
Cr-Commit-Position: refs/heads/main@{#976228}
27 files changed
tree: 722c47e5330c67d7ff012d792cc25dff8f1d0e65
  1. android_webview/
  2. apps/
  3. ash/
  4. base/
  5. build/
  6. build_overrides/
  7. buildtools/
  8. cc/
  9. chrome/
  10. chromecast/
  11. chromeos/
  12. cloud_print/
  13. codelabs/
  14. components/
  15. content/
  16. courgette/
  17. crypto/
  18. dbus/
  19. device/
  20. docs/
  21. extensions/
  22. fuchsia/
  23. gin/
  24. google_apis/
  25. google_update/
  26. gpu/
  27. headless/
  28. infra/
  29. ios/
  30. ipc/
  31. media/
  32. mojo/
  33. native_client_sdk/
  34. net/
  35. pdf/
  36. ppapi/
  37. printing/
  38. remoting/
  39. rlz/
  40. sandbox/
  41. services/
  42. skia/
  43. sql/
  44. storage/
  45. styleguide/
  46. testing/
  47. third_party/
  48. tools/
  49. ui/
  50. url/
  51. weblayer/
  52. .clang-format
  53. .clang-tidy
  54. .eslintrc.js
  55. .git-blame-ignore-revs
  56. .gitattributes
  57. .gitignore
  58. .gn
  59. .mailmap
  60. .rustfmt.toml
  61. .vpython
  62. .vpython3
  63. .yapfignore
  64. AUTHORS
  65. BUILD.gn
  66. CODE_OF_CONDUCT.md
  67. codereview.settings
  68. DEPS
  69. DIR_METADATA
  70. ENG_REVIEW_OWNERS
  71. LICENSE
  72. LICENSE.chromium_os
  73. OWNERS
  74. PRESUBMIT.py
  75. PRESUBMIT_test.py
  76. PRESUBMIT_test_mocks.py
  77. README.md
  78. WATCHLISTS
README.md

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