Ship the Credential Management API by default

This patch enables the Credential Management API by default.

It also converts the existing CLI flag into a `base::Feature`, which will allow
us to hook it up to Finch as a kill-switch in case it asplodes the internets.
This feature flag can be removed after ~a release, once we're confident that
we're not breaking things.

Intent to Ship: https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/7ouLjWzcjb0

BUG=576705
[email protected]

Review URL: https://codereview.chromium.org/1720403003

Cr-Commit-Position: refs/heads/master@{#377276}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index a9514540..5188ee9 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -1398,7 +1398,7 @@
      SINGLE_VALUE_TYPE(switches::kEnableTabAudioMuting)},
     {"enable-credential-manager-api", IDS_FLAGS_CREDENTIAL_MANAGER_API_NAME,
      IDS_FLAGS_CREDENTIAL_MANAGER_API_DESCRIPTION, kOsAll,
-     SINGLE_VALUE_TYPE(switches::kEnableCredentialManagerAPI)},
+     FEATURE_VALUE_TYPE(features::kCredentialManagementAPI)},
     {"reduced-referrer-granularity",
      IDS_FLAGS_REDUCED_REFERRER_GRANULARITY_NAME,
      IDS_FLAGS_REDUCED_REFERRER_GRANULARITY_DESCRIPTION, kOsAll,