Reduce the size of security_context.h

security_context.h is used in 2,700+ compilation units. crrev.com/594074
increased its pre-processed size by 1.32MB, and this CL fixes it.

Bug: 242216, 888668
Change-Id: I623ad22e5351328f934a0b1cd7caece6b4b07944
Reviewed-on: https://chromium-review.googlesource.com/1256470Reviewed-by: Daniel Bratell <bratell@opera.com>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595757}

third_party/blink/renderer/core/execution_context/security_context.h

@@ -29,7 +29,6 @@
 
 #include "base/macros.h"
 #include "base/memory/scoped_refptr.h"
-#include "third_party/blink/public/common/feature_policy/feature_policy.h"
 #include "third_party/blink/public/platform/web_insecure_request_policy.h"
 #include "third_party/blink/renderer/core/core_export.h"
 #include "third_party/blink/renderer/core/frame/sandbox_flags.h"
@@ -53,6 +52,7 @@ using ParsedFeaturePolicy = std::vector<ParsedFeaturePolicyDeclaration>;
 enum class ReportOptions { kReportOnFailure, kDoNotReport };
 
 namespace mojom {
+enum class FeaturePolicyFeature : int32_t;
 enum class IPAddressSpace : int32_t;
 }