[iOS] Fix accessibility: Sign-in button: color contrast ratios and narrator support

This CL improves the accessibility of the Sign-in button by adjusting
the text color to Blue 600 and setting the font size to 15. The
customization icon is set to size 17. Both the button and the icon use a
primary white background with 75% opacity. It also ensures that screen
readers (Narrator) now announce "Sign in options" when the button is
focused.

Screenshots for reference:

Narrator reads:
https://screenshot.googleplex.com/7VqEHAegwjiH4Vs

Old design (light):
https://screenshot.googleplex.com/54nPnmwBvrycK6G

Old design (dark):
N/A

New design (light):
https://screenshot.googleplex.com/KN6DT6v5LaaGi2b

New design (dark):
https://screenshot.googleplex.com/BrrmaC2ca8cmffc

Directional review:
https://docs.google.com/presentation/d/1lRln5BugIlsVbhjCybGrGjUuARTRK0kdS-sOGJ_y_a8/edit?slide=id.g35228ea0d01_0_11#slide=id.g35228ea0d01_0_11

Bug: 410599738

Change-Id: I977bc4ec2cf3571d57178d0332638707cfadce2a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6512449
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Pier-Alexandre Bouchard <pabouchard@google.com>
Reviewed-by: Scott Yoder <scottyoder@google.com>
Reviewed-by: Robbie Gibson <rkgibson@google.com>
Cr-Commit-Position: refs/heads/main@{#1457549}
4 files changed
tree: b9be5fcfa0f0878b4fc6544f730faed875f473c9
  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. crypto/
  16. dbus/
  17. device/
  18. docs/
  19. extensions/
  20. fuchsia_web/
  21. gin/
  22. google_apis/
  23. gpu/
  24. headless/
  25. infra/
  26. ios/
  27. ipc/
  28. media/
  29. mojo/
  30. native_client_sdk/
  31. net/
  32. pdf/
  33. ppapi/
  34. printing/
  35. remoting/
  36. rlz/
  37. sandbox/
  38. services/
  39. skia/
  40. sql/
  41. storage/
  42. styleguide/
  43. testing/
  44. third_party/
  45. tools/
  46. ui/
  47. url/
  48. webkit/
  49. .clang-format
  50. .clang-tidy
  51. .clangd
  52. .git-blame-ignore-revs
  53. .gitallowed
  54. .gitattributes
  55. .gitignore
  56. .gitmodules
  57. .gn
  58. .mailmap
  59. .rustfmt.toml
  60. .vpython3
  61. .yapfignore
  62. ATL_OWNERS
  63. AUTHORS
  64. BUILD.gn
  65. CODE_OF_CONDUCT.md
  66. codereview.settings
  67. CPPLINT.cfg
  68. CRYPTO_OWNERS
  69. DEPS
  70. DIR_METADATA
  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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