[flags] Make --js-flags settings have priority over V8 features

- Move --js-flags handling from the RenderProcessImpl constructor
  to a late stage in gin::V8Initializer::Initialize where they can
  override previously set flags
- Move content::switches::kJavaScriptFlags to blink::switches
- Allow blink_initializer.cc to access base::CommandLine to pass
  --js-flags to V8

V8 engineers are used to provide command-line flags manually.
For local testing it can be quite confusing if the flags conflict
with finch / field trial settings.
This CL changes the priority of --js-flags to be able to override
any flags previously set by features.

V8Flags are mostly initialized in two place:
1. content::RenderProcessImpl::RenderProcessImpl
2. gin::V8Initializer::Initialize

V8 flags are per-process globals, many of them should not be changed
after initialising V8. Thus the latest safe point to set V8 flags is
in gin::V8Initializer::Initialize.

Currently some flags are set in the RenderProcessImpl constructors,
especially --js-flags is processed there. With V8::SetFlagsFromString
the last flag takes precedence. As a result chrome feature flags that
are processed (late) in gin could not be overridden with --js-flags.

Bug: v8:12309
Change-Id: I832c274e760bf9bd61f9bf3fa8d7fc00251b4685
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3208413
Reviewed-by: Wez <wez@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Elad Alon <eladalon@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Finnur Thorarinsson <finnur@chromium.org>
Owners-Override: Wez <wez@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#931522}
27 files changed
tree: db0856993fe5c9da914349ef818d97c68c2e2782
  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. jingle/
  32. media/
  33. mojo/
  34. native_client_sdk/
  35. net/
  36. pdf/
  37. ppapi/
  38. printing/
  39. remoting/
  40. rlz/
  41. sandbox/
  42. services/
  43. skia/
  44. sql/
  45. storage/
  46. styleguide/
  47. testing/
  48. third_party/
  49. tools/
  50. ui/
  51. url/
  52. weblayer/
  53. .clang-format
  54. .clang-tidy
  55. .eslintrc.js
  56. .git-blame-ignore-revs
  57. .gitattributes
  58. .gitignore
  59. .gn
  60. .mailmap
  61. .rustfmt.toml
  62. .vpython
  63. .vpython3
  64. .yapfignore
  66. BUILD.gn
  68. codereview.settings
  69. DEPS
  73. LICENSE.chromium_os
  74. OWNERS
  75. PRESUBMIT.py
  76. PRESUBMIT_test.py
  77. PRESUBMIT_test_mocks.py
  78. README.md

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