Prototype an 'allow-unsandboxed-auxiliary' sandbox flag.

Source/core/dom/SandboxFlags.cpp

Index: Source/core/dom/SandboxFlags.cpp
diff --git a/Source/core/dom/SandboxFlags.cpp b/Source/core/dom/SandboxFlags.cpp
index 4df79990cba5112f93c3e5f9fd8fc20a19f26501..157685d4a21b773a51ef79f5287697282ff30cec 100644
--- a/Source/core/dom/SandboxFlags.cpp
+++ b/Source/core/dom/SandboxFlags.cpp
@@ -28,6 +28,7 @@
 #include "core/dom/SandboxFlags.h"
 
 #include "core/html/parser/HTMLParserIdioms.h"
+#include "platform/RuntimeEnabledFeatures.h"
 #include "wtf/text/StringBuilder.h"
 
 namespace blink {
@@ -65,6 +66,8 @@ SandboxFlags parseSandboxPolicy(const SpaceSplitString& policy, String& invalidT
             flags &= ~SandboxPointerLock;
         } else if (equalIgnoringCase(sandboxToken, "allow-orientation-lock")) {
             flags &= ~SandboxOrientationLock;
+        } else if (equalIgnoringCase(sandboxToken, "allow-unsandboxed-auxiliary") && RuntimeEnabledFeatures::unsandboxedAuxiliaryEnabled()) {
+            flags &= ~SandboxPropagatesToAuxiliaryBrowsingContexts;
         } else {
             if (numberOfTokenErrors)
                 tokenErrors.appendLiteral(", '");