Add a CustomSchemeHandler runtime flag to remove ENABLE(CUSTOM_SCHEME_HANDLER)

Source/web/ChromeClientImpl.cpp

Index: Source/web/ChromeClientImpl.cpp
diff --git a/Source/web/ChromeClientImpl.cpp b/Source/web/ChromeClientImpl.cpp
index 3f66479096757659e797f53bf0561f063d469b59..ec7b15d759e0ec26c9d8a69ddf315091ebd0edde 100644
--- a/Source/web/ChromeClientImpl.cpp
+++ b/Source/web/ChromeClientImpl.cpp
@@ -91,6 +91,7 @@
 #include "platform/Cursor.h"
 #include "platform/DateTimeChooser.h"
 #include "platform/FileChooser.h"
+#include "platform/NotImplemented.h"
 #include "platform/PlatformScreen.h"
 #include "platform/exported/WrappedResourceRequest.h"
 #include "platform/geometry/FloatRect.h"
@@ -1030,4 +1031,15 @@ void NavigatorContentUtilsClientImpl::registerProtocolHandler(const String& sche
     m_webView->client()->registerProtocolHandler(scheme, baseURL, url, title);
 }
 
+NavigatorContentUtilsClient::CustomHandlersState NavigatorContentUtilsClientImpl::isProtocolHandlerRegistered(const String& scheme, const String& baseURL, const String& url)
+{
+    notImplemented();
+    return NavigatorContentUtilsClient::CustomHandlersDeclined;
+}
+
+void NavigatorContentUtilsClientImpl::unregisterProtocolHandler(const String& scheme, const String& baseURL, const String& url)
+{
+    notImplemented();
+}
+
 } // namespace blink