Introduce SignedExchangeReportingForDistributors feature flag

I will implement Signed Exchange Reporting for distributors behind this flag.
https://www.chromestatus.com/features/5687904902840320

Bug: 910516
Change-Id: Ic96c1e13e00c94a524123322cb58201115de1add
Reviewed-on: https://chromium-review.googlesource.com/c/1459875Reviewed-by: Kunihiko Sakamoto <ksakamoto@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630303}

content/public/common/content_features.cc

@@ -423,6 +423,12 @@ const base::Feature kSharedArrayBuffer {
 #endif
 };
 
+// Signed Exchange Reporting for distributors
+// https://www.chromestatus.com/features/5687904902840320
+const base::Feature kSignedExchangeReportingForDistributors{
+    "SignedExchangeReportingForDistributors",
+    base::FEATURE_DISABLED_BY_DEFAULT};
+
 // Origin-Signed HTTP Exchanges (for WebPackage Loading)
 // https://www.chromestatus.com/features/5745285984681984
 const base::Feature kSignedHTTPExchange{"SignedHTTPExchange",

content/public/common/content_features.h

@@ -101,6 +101,8 @@ CONTENT_EXPORT extern const base::Feature kSecMetadata;
 CONTENT_EXPORT extern const base::Feature kServiceWorkerLongRunningMessage;
 CONTENT_EXPORT extern const base::Feature kServiceWorkerPaymentApps;
 CONTENT_EXPORT extern const base::Feature kSharedArrayBuffer;
+CONTENT_EXPORT extern const base::Feature
+    kSignedExchangeReportingForDistributors;
 CONTENT_EXPORT extern const base::Feature kSignedHTTPExchange;
 CONTENT_EXPORT extern const base::Feature kSignedHTTPExchangeAcceptHeader;
 CONTENT_EXPORT extern const char