[Extensions MV2] Emit UKM when a user re-enables an MV2 extension

Chrome has announced that MV2 extensions will gradually no longer be
supported in the browser. See more information at
https://developer.chrome.com/blog/resuming-the-transition-to-mv3.
The second step of this, after warning users about MV2 extensions, will
be to disable unsupported MV2 extensions. If users want to re-enable
these extensions, they will be able to.

This CL adds UKM to be emitted when the user re-enables an MV2
extension, along with tests for the same.

UKM Doc (internal only): https://docs.google.com/document/d/1bj31UE30ZmYo-qQAbqi1Gvoo5FHbm9gIb8BO63SQtD0/edit?resourcekey=0-pG2HEFa2SwPgnlKlLvPJ7g&tab=t.0#heading=h.1wkdyavg5a1n

Bug: 339061151
Change-Id: I3fdfd1e8ce9616a02b4a33e48982a8f599a92ff8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5731681
Commit-Queue: Devlin Cronin <rdevlin.cronin@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1338099}
7 files changed
tree: 7fe2db731c29ee00b8751300902d11bf2cf3b331
  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. webkit/
  51. .clang-format
  52. .clang-tidy
  53. .clangd
  54. .eslintrc.js
  55. .git-blame-ignore-revs
  56. .gitallowed
  57. .gitattributes
  58. .gitignore
  59. .gitmodules
  60. .gn
  61. .mailmap
  62. .rustfmt.toml
  63. .vpython3
  64. .yapfignore
  65. ATL_OWNERS
  66. AUTHORS
  67. BUILD.gn
  68. CODE_OF_CONDUCT.md
  69. codereview.settings
  70. CPPLINT.cfg
  71. DEPS
  72. DIR_METADATA
  73. LICENSE
  74. LICENSE.chromium_os
  75. OWNERS
  76. PRESUBMIT.py
  77. PRESUBMIT_test.py
  78. PRESUBMIT_test_mocks.py
  79. README.md
  80. WATCHLISTS
README.md

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