Adds EnableReadingList experimental flag.

ios/chrome/browser/experimental_flags.mm

Index: ios/chrome/browser/experimental_flags.mm
diff --git a/ios/chrome/browser/experimental_flags.mm b/ios/chrome/browser/experimental_flags.mm
index ccbe26b6792b55cc7e1f7ee45cbc2a413912e60b..21f8828069cd5a562902373c890463cef7bd3f0d 100644
--- a/ios/chrome/browser/experimental_flags.mm
+++ b/ios/chrome/browser/experimental_flags.mm
@@ -28,6 +28,7 @@ NSString* const kEnableViewCopyPasswords = @"EnableViewCopyPasswords";
 NSString* const kHeuristicsForPasswordGeneration =
     @"HeuristicsForPasswordGeneration";
 const char* const kWKWebViewTrialName = "IOSUseWKWebView";
+NSString* const kEnableReadingList = @"EnableReadingList";
 
 enum class WKWebViewEligibility {
   // UNSET indicates that no explicit call to set eligibility has been made,
@@ -238,4 +239,8 @@ bool IsTabSwitcherEnabled() {
                           base::CompareCase::INSENSITIVE_ASCII);
 }
 
+bool IsReadingListEnabled() {
+  return [[NSUserDefaults standardUserDefaults] boolForKey:kEnableReadingList];
+}
+
 }  // namespace experimental_flags