[Sync::USS] Allow Sync USS Passwords for Linux

Sync USS Password on Linux has been unblocked by recent migrations
in the PasswordManager team that are concluded in the parent CL.
This CL is enabling the about flags for Linux and it enables it by
default for all linux bots.

Change-Id: Ic94267ab0e9e4cce2ac2b02f1dfa9a0a9046e902
Bug: 902349
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1718332
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681044}

chrome/browser/about_flags.cc

@@ -3778,8 +3778,7 @@ const FeatureEntry kFeatureEntries[] = {
      SINGLE_VALUE_TYPE(switches::kDisableBestEffortTasks)},
     {"enable-sync-uss-passwords",
      flag_descriptions::kEnableSyncUSSPasswordsName,
-     flag_descriptions::kEnableSyncUSSPasswordsDescription,
-     kOsMac | kOsWin | kOsCrOS | kOsAndroid,
+     flag_descriptions::kEnableSyncUSSPasswordsDescription, kOsAll,
      FEATURE_VALUE_TYPE(switches::kSyncUSSPasswords)},
 
 #if defined(OS_WIN) || defined(OS_MACOSX) || defined(OS_CHROMEOS)

testing/variations/fieldtrial_testing_config.json

@@ -5234,6 +5234,7 @@
                 "android",
                 "chromeos",
                 "ios",
+                "linux",
                 "mac",
                 "windows"
             ],