Introduce about flag for USS session sync

This allows more easy testing of the feature under development/
experimentation.

Bug: 681921
Change-Id: Ibdf822b04312e794ec2a20051d02f1847275a9f9
Reviewed-on: https://chromium-review.googlesource.com/1024831
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553318}

chrome/browser/about_flags.cc

@@ -3875,6 +3875,9 @@ const FeatureEntry kFeatureEntries[] = {
      kOsAll,
      FEATURE_VALUE_TYPE(
          autofill::features::kAutofillEnforceMinRequiredFieldsForUpload)},
+    {"enable-sync-uss-sessions", flag_descriptions::kEnableSyncUSSSessionsName,
+     flag_descriptions::kEnableSyncUSSSessionsDescription, kOsAll,
+     FEATURE_VALUE_TYPE(switches::kSyncUSSSessions)},
 
     // NOTE: Adding a new flag requires adding a corresponding entry to enum
     // "LoginCustomFlags" in tools/metrics/histograms/enums.xml. See "Flag

chrome/browser/flag_descriptions.cc

@@ -607,6 +607,11 @@ const char kEnablePixelCanvasRecordingDescription[] =
     "with the pixel and improves text rendering. This should be enabled when a "
     "device is using fractional scale factor.";
 
+const char kEnableSyncUSSSessionsName[] = "Enable USS for sessions sync";
+const char kEnableSyncUSSSessionsDescription[] =
+    "Enables the new, experimental implementation of session sync (aka tab "
+    "sync).";
+
 const char kEnableTokenBindingName[] = "Token Binding.";
 const char kEnableTokenBindingDescription[] = "Enable Token Binding support.";
 

chrome/browser/flag_descriptions.h

@@ -386,6 +386,9 @@ extern const char kEnablePixelCanvasRecordingDescription[];
 extern const char kEnablePWAFullCodeCacheName[];
 extern const char kEnablePWAFullCodeCacheDescription[];
 
+extern const char kEnableSyncUSSSessionsName[];
+extern const char kEnableSyncUSSSessionsDescription[];
+
 extern const char kEnableTokenBindingName[];
 extern const char kEnableTokenBindingDescription[];
 

tools/metrics/histograms/enums.xml

@@ -27458,6 +27458,7 @@ from previous Chrome versions.
   <int value="955340765" label="ChromeHomeOptOutSnackbar:enabled"/>
   <int value="963457392" label="ChromeHomeModernLayout:disabled"/>
   <int value="963671232" label="DrawOcclusion:disabled"/>
+  <int value="972228058" label="SyncUSSSessions:disabled"/>
   <int value="975104092" label="show-taps"/>
   <int value="979445973" label="OmniboxSpareRenderer:enabled"/>
   <int value="980396200" label="enable-new-korean-ime"/>
@@ -27819,6 +27820,7 @@ from previous Chrome versions.
   <int value="1789517771" label="MacV2Sandbox:enabled"/>
   <int value="1792609232" label="NTPShortcuts:enabled"/>
   <int value="1803465156" label="enable-zero-suggest-most-visited"/>
+  <int value="1803470125" label="SyncUSSSessions:enabled"/>
   <int value="1809940714" label="SpeculativeLaunchServiceWorker:disabled"/>
   <int value="1812368073" label="enable-new-app-list-mixer"/>
   <int value="1814671708" label="disable-password-manager-reauthentication"/>