[Periodic Background Sync] Add flag to enable it.

This adds logic to enable Periodic Background Sync through chrome
flags, so that early adapters can enable it for testing.

Bug: 925297
Change-Id: I6fbb2ce68fce254b43cb6a702a7f44737e7668cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1648218
Reviewed-by: Peter Beverloo <[email protected]>
Commit-Queue: Mugdha Lakhani <[email protected]>
Cr-Commit-Position: refs/heads/master@{#667580}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 89ae12f..45c7b6f 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -3908,6 +3908,9 @@
      flag_descriptions::kEnableAutofillCreditCardUploadFeedbackDescription,
      kOsWin | kOsMac | kOsLinux,
      FEATURE_VALUE_TYPE(autofill::features::kAutofillCreditCardUploadFeedback)},
+    {"periodic-background-sync", flag_descriptions::kPeriodicBackgroundSyncName,
+     flag_descriptions::kPeriodicBackgroundSyncDescription, kOsAll,
+     FEATURE_VALUE_TYPE(features::kPeriodicBackgroundSync)},
 
     // NOTE: Adding a new flag requires adding a corresponding entry to enum
     // "LoginCustomFlags" in tools/metrics/histograms/enums.xml. See "Flag