Add histograms_index file to record all available histograms xmls

This histograms_index file provides a convenient way for a job to know
all of the histogram files without having to search for them. As we
split up histograms, we need a way to locate all available histograms
xmls and thus we need this index file.

This cl also adds a PRESUBMIT check to make sure this index file is
up-to-date.

Bug: 1116807
Change-Id: Ifc55e421092448b5cecaa573f02e651a098428bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2359234
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: Weilun Shi <sweilun@chromium.org>
Auto-Submit: Weilun Shi <sweilun@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800357}
4 files changed
tree: 264797dae0eaeabe01537af782c31867410226d4
  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. DIR_METADATA
  68. ENG_REVIEW_OWNERS
  69. LICENSE
  70. LICENSE.chromium_os
  71. OWNERS
  72. PRESUBMIT.py
  73. PRESUBMIT_test.py
  74. PRESUBMIT_test_mocks.py
  75. README.md
  76. WATCHLISTS
README.md

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