Bug 1811294: Roll out Pref Sanitization r=smaug

1. Ship the Sanitization (currently on in Nightly
     but without crashing) to the trains
2. Enabling crashing in Nightly

#1 will start omitting the preference values in
Content Processes. An affected user will have
their browser behave differently - it will be as
if the preference has no value. So for e.g. the
font preferences, their font customizations will
be gone. If they file a bug about this happening
and an engineer connects the problem to pref
sanitization we could debug the issue and maybe
understand it.

#2 will mean that an affected user (of which
there are currently none we think) will have a
content process crash when they access a
forbidden preference. The crash reason will
contain the name of the preference, and the
stack which would help us better understand
why we crashed.

Differential Revision: https://phabricator.services.mozilla.com/D167287

modules/libpref/Preferences.cpp

@@ -6002,7 +6002,7 @@ struct PrefListEntry {
   size_t mLen;
 };
 
-// These prefs are not useful in child processes.
+// These prefs are not useful in child processes - do not send them
 static const PrefListEntry sParentOnlyPrefBranchList[] = {
     // Remove prefs with user data
     PREF_LIST_ENTRY("datareporting.policy."),
@@ -6051,6 +6051,9 @@ static const PrefListEntry sParentOnlyPrefBranchList[] = {
     PREF_LIST_ENTRY("toolkit.telemetry.previousBuildID"),
 };
 
+// These prefs are dynamically-named (i.e. not specified in prefs.js or
+// StaticPrefList) and would normally by blocklisted but we allow them through
+// anyway, so this override list acts as an allowlist
 static const PrefListEntry sDynamicPrefOverrideList[]{
     PREF_LIST_ENTRY("apz.subtest"),
     PREF_LIST_ENTRY("autoadmin.global_config_url"),  // Bug 1780575

modules/libpref/init/StaticPrefList.yaml

@@ -5168,13 +5168,13 @@
 # If true, do not send blocklisted preference values to the subprocess
 - name: fission.omitBlocklistedPrefsInSubprocesses
   type: RelaxedAtomicBool
-  value: @IS_NIGHTLY_BUILD@
+  value: true
   mirror: always
 
 # If true, crash when a blocklisted preference is accessed in a subprocess
 - name: fission.enforceBlocklistedPrefsInSubprocesses
   type: RelaxedAtomicBool
-  value: false
+  value: @IS_NIGHTLY_BUILD@
   mirror: always
 
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