[initial-letter] Layout tree support for CSS initial-letter property

This patch introduces CSS initial letter support for layout tree as
preparation of implementing layouting initial-letter[1].

The "initial-letter" property affcts layout tree as below:
 * Introduce `LayoutObject::IsInitialLetterBox()` to identify whether
   it is initial letter box or not.
 * Adjusting `ComputedStyle` for `<::first-letter>` and its text for
   layout.
    - The initial letter box uses atomic inline block for positioning
      and layout isolation from surronding text.

[1] http://crrev.com/c/3981353: [initial-letter] Layout implementation

Bug: 1276900
Change-Id: I8903518e97df00aba282f07019e69c598c7b3c6d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3985013
Auto-Submit: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1064800}
10 files changed
tree: 97e937fff9d1ff7d1daec646df24b13f23161ebd
  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. codelabs/
  13. components/
  14. content/
  15. courgette/
  16. crypto/
  17. dbus/
  18. device/
  19. docs/
  20. extensions/
  21. fuchsia_web/
  22. gin/
  23. google_apis/
  24. google_update/
  25. gpu/
  26. headless/
  27. infra/
  28. ios/
  29. ipc/
  30. media/
  31. mojo/
  32. native_client_sdk/
  33. net/
  34. pdf/
  35. ppapi/
  36. printing/
  37. remoting/
  38. rlz/
  39. sandbox/
  40. services/
  41. skia/
  42. sql/
  43. storage/
  44. styleguide/
  45. testing/
  46. third_party/
  47. tools/
  48. ui/
  49. url/
  50. weblayer/
  51. .clang-format
  52. .clang-tidy
  53. .eslintrc.js
  54. .git-blame-ignore-revs
  55. .gitattributes
  56. .gitignore
  57. .gn
  58. .mailmap
  59. .rustfmt.toml
  60. .vpython3
  61. .yapfignore
  62. AUTHORS
  63. BUILD.gn
  64. CODE_OF_CONDUCT.md
  65. codereview.settings
  66. DEPS
  67. DIR_METADATA
  68. ENG_REVIEW_OWNERS
  69. LICENSE
  70. LICENSE.chromium_os
  71. OWNERS
  72. PRESUBMIT.py
  73. PRESUBMIT_test.py
  74. PRESUBMIT_test_mocks.py
  75. README.md
  76. WATCHLISTS
README.md

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