[Preconnect] Stop specially prioritizing preconnect tasks

This CL eliminates the execution of preconnect tasks on a custom
high-priority task queue that was introduced in
https://chromium-review.googlesource.com/c/chromium/src/+/1527536,
reverting back to the behavior prior to that CL of executing these tasks
on the default (priority-based) task queues. We have done
experimentation and found that execution on the custom high-priority
task queue brought no performance benefits over execution on the default
priority-based task queues (summary in
https://bugs.chromium.org/p/chromium/issues/detail?id=1257582#c28).

Specifically, this CL is implementing the launch of the NoStandaloneQ
arm of the experiment analyzed on that bug. The logic for why this is
the case bears explicit mention:

- NoStandaloneQ enables the "kTreatPreconnectTaskTypeAsDefault"
  base::Feature.
- When this feature is enabled, invoking
  GetUIThreadTaskRunner({BrowserTaskType::kPreconnect}) returns the
  task runner for the task priority of the BrowserTaskTraits instance
  created via {BrowserTaskType::kPreconnect}. This task priority is
  USER_BLOCKING, since BrowserTaskTraits inherits the default value
  from TaskTraits and doesn't modify that default value.
- In this CL, we simply stop passing a TaskRunner to the calls to
  BindNewPipeAndPassRemote() made in the preconnect code. When no
  TaskRunner is explicitly specified to these calls, they use
  SequencedTaskRunnerHandle::Get(). The task runner returned by
  SequencedTaskRunnerHandle::Get() on the UI thread (on which these
  calls are executing) is also the task runner for the USER_BLOCKING
  priority, i.e., the same task runner that is currently being passed
  when kTreatPreconnectTaskTypeAsDefault is enabled.

This CL also eliminates the kPreconnect browser task trait, as there
are now no uses of it in the codebase.

Bug: 1257582
Change-Id: Idecfb145cd3299c38e5e479385e5dccdab2e0056
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3698086
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1015858}
9 files changed
tree: 2b1892c6a6e8773917b2697f7eac02fb22273fc7
  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/
  22. fuchsia_web/
  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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