Add Chrome.Flags.CachedFlag.* KeyPrefix for FeatureUtilities.

It is not currently used, but needs to be there for when a new
feature flag is cached.

Bug: 1022108,1012975

Change-Id: I759940be55dc5ae3bb2d72207a1e376a491324f0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1970370
Commit-Queue: Henrique Nakashima <hnakashima@chromium.org>
Reviewed-by: Theresa  <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726146}

chrome/android/java/src/org/chromium/chrome/browser/flags/FeatureUtilities.java

@@ -42,14 +42,13 @@ import java.util.Map;
  * startup before native is initialized but are set via native code. The caching is done in
  * {@link android.content.SharedPreferences}, which is available in Java immediately.
  *
- * To add a new cached flag:
- * - Create a constant to act as SharedPreferences key in {@link ChromePreferenceKeys}, with the
- *   value "Chrome.Flags.FooEnabled".
+ * To cache the flag {@link ChromeFeatureList}.FOO:
  * - Call {@link FeatureUtilities#cacheFlag(String, String)} in {@link
- *   FeatureUtilities#cacheNativeFlags()} passing the new key create above and the {@link
- *   ChromeFeatureList} flag.
+ *   FeatureUtilities#cacheNativeFlags()} passing
+ *   {@link ChromePreferenceKeys#CACHED_FEATURE_FLAG}.createKey(FOO) and FOO.
  * - To query whether the cached flag is enabled in client code, call
- *   {@link FeatureUtilities#isFlagEnabled(String, boolean)} passing the SharedPreferences key.
+ *   {@link FeatureUtilities#isFlagEnabled(String, boolean)} passing
+ *   {@link ChromePreferenceKeys#CACHED_FEATURE_FLAG}.createKey(FOO).
  *   Consider this the source of truth for whether the flag is turned on in the current session.
  * - When querying whether a cached feature is enabled from native, a @CalledByNative method can be
  *   exposed in this file to allow feature_utilities.cc to retrieve the cached value.

chrome/browser/preferences/android/java/src/org/chromium/chrome/browser/preferences/ChromePreferenceKeys.java

@@ -343,6 +343,11 @@ public final class ChromePreferenceKeys {
     // Keys representing cached feature flags                                                     //
     ////////////////////////////////////////////////////////////////////////////////////////////////
 
+    /**
+     * Cached feature flags generated by FeatureUtilities use this prefix.
+     */
+    public static final KeyPrefix CACHED_FEATURE_FLAG = new KeyPrefix("Chrome.Flags.CachedFlag.*");
+
     /**
      * Key for whether DownloadResumptionBackgroundTask should load native in service manager only
      * mode.
@@ -569,6 +574,7 @@ public final class ChromePreferenceKeys {
                 DATA_REDUCTION_FIRST_ENABLED_TIME,
 
                 // Cached feature flags
+                CACHED_FEATURE_FLAG.pattern(),
                 SERVICE_MANAGER_FOR_DOWNLOAD_RESUMPTION_KEY,
                 SERVICE_MANAGER_FOR_BACKGROUND_PREFETCH_KEY,
                 INTEREST_FEED_CONTENT_SUGGESTIONS_KEY,