flags: document expiry process & M77 expiry set

Bug: 953690
Change-Id: I155609bda62bb9170c543f671bcda85dd90af0b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1708830
Auto-Submit: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681931}

docs/flag_expiry.md

+# Flag Expiry
+
+This document outlines the process by which flags in chromium expire and are
+removed from the codebase, and describes which flags are about to expire. This
+is the authoritative list of flags that are expiring and being removed. This
+document only describes entries in chrome://flags, *not* command-line switches
+(commonly also called command-line flags). This process does not cover
+command-line switches and there continue to be no guarantees about those.
+
+[TOC]
+
+## Do Not Depend On Flags
+
+If you are using (or think you need to use) a flag to configure Chromium for
+your use case, please [file a bug] or email flags-dev@, because that flag will
+likely be removed at some point.
+
+Flags have never been a supported configuration surface in Chromium, and we have
+never guaranteed that any specific flag will behave consistently or even
+continue to exist. This document describes a process for removing flags that
+have been around for long enough that users *might* have come to rely on their
+ongoing existence in a way that hopefully minimizes pain, but Chromium
+developers are free to change the behavior of or remove flags at any time. In
+particular, just because a flag will expire through this process does not mean a
+developer will not remove it earlier than this process specifies.
+
+## The Process
+
+After each milestone's branch point:
+
+1. The flags team chooses a set of flags to begin expiring, from the list
+   produced by `tools/flags/list-flags.py --expired-by $MSTONE`. In the steady
+   state, when there is not a big backlog of flags to remove, this set will be
+   the entire list of flags that are `expired-by $MSTONE`.
+2. The flags team hides the flags in this set by default from `chrome://flags`,
+   and adds a flag `temporary-unexpire-flags-M$MSTONE` and a base::Feature
+   `TemporaryUnexpireFlagsM$MSTONE` which unhide these flags. When hidden from
+   `chrome://flags`, all the expired flags will behave as if unset, so users
+   cannot be stuck with a non-default setting of a hidden flag.
+3. After two further milestones have passed (i.e. at $MSTONE+2 branch), the
+   temporary unhide flag & feature will be removed (meaning the flags are now
+   permanently invisible), and TPMs will file bugs against the listed owners to
+   remove the flags and clean up the backing code.
+
+There are other elements of this process not described here, such as emails to
+flags-dev@ tracking the status of the process.
+
+## The Set
+
+In M77, the following flags are being hidden as the second step of this process.
+If you are using one of these flags for some reason, please get in touch with
+the flags team (via flags-dev@) and/or the listed owner(s) of that flag. This
+list will be updated at each milestone as we expire more flags. This is the
+authoritative source of the expiry set for a given milestone.
+
+* allow-remote-context-for-notifications
+* allow-starting-service-manager-only
+* android-files-in-files-app
+* app-service-ash
+* arc-available-for-child
+* arc-boot-completed-broadcast
+* arc-custom-tabs-experiment
+* arc-documents-provider
+* arc-graphics-buffer-visualization-tool
+* arc-native-bridge-experiment
+* arc-usb-host
+* arc-vpn
+* ash-enable-pip-rounded-corners
+* ash-notification-stacking-bar-redesign
+* autofill-always-show-server-cards-in-sync-transport
+* autofill-dynamic-forms
+* autofill-enable-company-name
+* autofill-enable-local-card-migration-for-non-sync-user
+* autofill-enforce-min-required-fields-for-heuristics
+* autofill-enforce-min-required-fields-for-query
+* autofill-enforce-min-required-fields-for-upload
+* autofill-restrict-formless-form-extraction
+* autofill-rich-metadata-queries
+* autofill-settings-split-by-card-type
+* background-task-component-update
+* calculate-native-win-occlusion
+* cct-module-cache
+* cct-module-custom-request-header
+* cct-module-dex-loading
+* cct-module-post-message
+* cct-module-use-intent-extras
+* contextual-search
+* crostini-usb-support
+* cryptauth-v2-enrollment
+* document-passive-wheel-event-listeners
+* enable-app-list-search-autocomplete
+* enable-arc-cups-api
+* enable-arc-unified-audio-focus
+* enable-assistant-dsp
+* enable-assistant-stereo-input
+* enable-assistant-voice-match
+* enable-autofill-credit-card-upload-editable-cardholder-name
+* enable-autofill-credit-card-upload-editable-expiration-date
+* enable-autofill-do-not-upload-save-unsupported-cards
+* enable-autofill-import-dynamic-forms
+* enable-autofill-import-non-focusable-credit-card-forms
+* enable-autofill-local-card-migration-uses-strike-system-v2
+* enable-autofill-save-card-improved-user-consent
+* enable-autofill-send-experiment-ids-in-payments-rpcs
+* enable-bloated-renderer-detection
+* enable-bulk-printers
+* enable-chromeos-account-manager
+* enable-custom-mac-paper-sizes
+* enable-encryption-migration
+* enable-experimental-accessibility-features
+* enable-experimental-accessibility-language-detection
+* enable-fs-nosymfollow
+* enable-google-branded-context-menu
+* enable-immersive-fullscreen-toolbar
+* enable-improved-geolanguage-data
+* enable-myfiles-volume
+* enable-native-controls
+* enable-native-google-assistant
+* enable-reopen-tab-in-product-help
+* enable-safe-browsing-ap-download-verdicts
+* enable-vaapi-jpeg-image-decode-acceleration
+* enable-webassembly
+* enable-webrtc-hw-vp9-encoding
+* enable-webrtc-pipewire-capturer
+* enable-zero-state-suggestions
+* enable_messages_web_push
+* force-use-chrome-camera
+* foreground-notification-manager
+* gdi-text-printing
+* handwriting-gesture
+* manual-password-generation-android
+* network-service
+* offline-indicator-always-http-probe
+* offline-pages-ct-suppress-completed-notification
+* offline-pages-load-signal-collecting
+* offline-pages-resource-based-snapshot
+* omnibox-experimental-keyword-mode
+* on-the-fly-mhtml-hash-computation
+* passwords-keyboard-accessory
+* passwords-migrate-linux-to-login-db
+* pdf-annotations
+* postscript-printing
+* remove-ntp-fakebox
+* rewrite-leveldb-on-deletion
+* session-restore-prioritizes-background-use-cases
+* smart-text-selection
+* stop-in-background
+* sync-USS-autofill-wallet-metadata
+* ui-show-composited-layer-borders
+* unfiltered-bluetooth-devices
+* unsafely-treat-insecure-origin-as-secure
+* use_messages_google_com_domain
+* use_messages_staging_url
+* views-cast-dialog
+* wake-on-wifi-packet
+
+[file a bug]: https://new.crbug.com