Write last_modified date to Content Settings in the PrefProvider

Introduce a last_modified timestamp for content settings stored inside the PrefProvider.
This timestamp will enable deleting content-settings by a selected time range from the clear browsing data dialog.
The timestamp will only be recorded if the tabs-in-cbd flag is activated because otherwise there would be no way to delete these history-like entries.

BUG=681523

Review-Url: https://codereview.chromium.org/2812113004
Cr-Commit-Position: refs/heads/master@{#468950}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 25a5da5..0a423b8 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -2340,7 +2340,7 @@
      FEATURE_VALUE_TYPE(chrome::android::kImportantSitesInCBD)},
     {"tabs-in-cbd", flag_descriptions::kTabsInCbdName,
      flag_descriptions::kTabsInCbdDescription, kOsAndroid,
-     FEATURE_VALUE_TYPE(chrome::android::kTabsInCBD)},
+     FEATURE_VALUE_TYPE(features::kTabsInCbd)},
 #endif  // OS_ANDROID
     {"enable-pointer-events", flag_descriptions::kExperimentalPointerEventName,
      flag_descriptions::kExperimentalPointerEventDescription, kOsAll,