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This change implements flag expiration, with the same goal but a very different method than the previous implementation (https://chromium-review.googlesource.com/c/chromium/src/+/1729449). This change preserves the flag-metadata.json file as the sole authoritative source for flag expiration data. During the build process, a new script (//tools/flags/generate_expired_list.py) generates a data structure from flag-metadata.json listing flags that are expired at or before the current milestone, with their expiration milestones. Logic in //chrome/browser/about_flags.cc and //chrome/browser/unexpire_flags.cc then conditionally hides these flags, depending on the state of the unexpire flags - see //doc/flag_expiry.md for details about those. Specifically, this change: 1) Renames //tools/flags/list-flags.py to list_flags.py to make it importable as a module; 2) Introduces //chrome/browser/expired_flags_list.h, which describes a data structure for representing expired flags; 3) Introduces //tools/flags/generate_expired_list.py, which takes as inputs //chrome/browser/flag-metadata.json and //chrome/VERSION and generates a C++ source file containing a data structure matching that in expired_flags_list.h; 4) Has the //chrome/browser build target depend on the generated file from step 3; 5) Re-introduces an unexpire flag as described in //doc/flag_expiry.md; 6) Adds logic to //chrome/browser/about_flags.cc to conditionally hide expired flags. This means that flag-metadata.json is the sole determinant of whether a flag is currently expired or not. Note that this change expires all flags whose expiration milestones were *M76* or earlier, not M78 (the current mstone); this is because the backlog of flags to expire is currently quite large. In M79, we will expire flags <= M78, then in M80, we will expire flags <= M80; after that, each milestone Mx will expire flags <= Mx. Also note that this change discards the notion of flag expiry depending on usage metrics - i.e., we will now expire *all* flags that expire before the target milestone, not just the N with the lowest usage. The lowest-usage design proved to be extremely manual to implement and very difficult for developers to predict the results of. Bug: 953690 Change-Id: I7389b667d1e032c795c137cafadf0e5e80fb82c1 Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1754762 Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org> Reviewed-by: Nico Weber <thakis@chromium.org> Cr-Commit-Position: refs/heads/master@{#690742}
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