[FragmentItem] Make NGFragmentItem not a DisplayItemClient

Making |NGFragmentItem| not a |DisplayItemClient| has two
benefits:
1. Give a persistent |DisplayItemClientId| across multiple
   layout cycles.
2. Allow making |Vector<NGFragmentItem>| instead of
   |Vector<scoped_refptr<NGFragmentItem>>|, because a
   |Vector| of |DisplayItemClient| is not allowed.
   This change is expected to reduce the memory allocation
   cost, which is high for text-heavy pages.

This patch is only for 1. The work for 2 will be in following
patches. Still, this patch improves `blink_perf.paint/
paint-offset-changes` microbenchmark by ~10%:
https://pinpoint-dot-chromeperf.appspot.com/job/15b8f60a120000
https://pinpoint-dot-chromeperf.appspot.com/job/14e61bb6120000
and slight positive changes to blink_perf.layout, probably
due to simpler paint invalidations:
https://pinpoint-dot-chromeperf.appspot.com/job/17d3126c120000

This patch is on top of following foudnation patches:
* r770260 <crrev.com/c/2207948> supported |wtf_size_t| for
  fragment id.
* r770637 <crrev.com/c/2209774> added |NGFragment::
  FragmentId()|.

Bug: 982194
Change-Id: I1cb5e2633bd591723632f1acb0cbb09ab4d77f83
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2208586
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771601}
10 files changed
tree: 09beb0d4fcf96a9d9bb7f8c0562765eb544ccd5f
  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. components/
  14. content/
  15. courgette/
  16. crypto/
  17. dbus/
  18. device/
  19. docs/
  20. extensions/
  21. fuchsia/
  22. gin/
  23. google_apis/
  24. google_update/
  25. gpu/
  26. headless/
  27. infra/
  28. ios/
  29. ipc/
  30. jingle/
  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. .vpython
  60. .vpython3
  61. .yapfignore
  62. AUTHORS
  63. BUILD.gn
  64. CODE_OF_CONDUCT.md
  65. codereview.settings
  66. DEPS
  67. ENG_REVIEW_OWNERS
  68. LICENSE
  69. LICENSE.chromium_os
  70. OWNERS
  71. PRESUBMIT.py
  72. PRESUBMIT_test.py
  73. PRESUBMIT_test_mocks.py
  74. README.md
  75. WATCHLISTS
README.md

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