bpf_dsl: Move {Bool,Result}ExprImpl definition into separate header

sandbox/linux/bpf_dsl/bpf_dsl.cc

Index: sandbox/linux/bpf_dsl/bpf_dsl.cc
diff --git a/sandbox/linux/bpf_dsl/bpf_dsl.cc b/sandbox/linux/bpf_dsl/bpf_dsl.cc
index dd8eab6c140137217500e64fc32ac415a5ab1afb..63050ebcddc263677de5d7c20b9d06e60fe80e9c 100644
--- a/sandbox/linux/bpf_dsl/bpf_dsl.cc
+++ b/sandbox/linux/bpf_dsl/bpf_dsl.cc
@@ -10,6 +10,7 @@
 
 #include "base/logging.h"
 #include "base/memory/ref_counted.h"
+#include "sandbox/linux/bpf_dsl/bpf_dsl_impl.h"
 #include "sandbox/linux/seccomp-bpf/errorcode.h"
 #include "sandbox/linux/seccomp-bpf/sandbox_bpf.h"
 #include "sandbox/linux/seccomp-bpf/syscall_iterator.h"
@@ -404,3 +405,6 @@ ResultExpr SandboxBPFDSLPolicy::Trap(Trap::TrapFnc trap_func, const void* aux) {
 
 }  // namespace bpf_dsl
 }  // namespace sandbox
+
+template class scoped_refptr<const sandbox::bpf_dsl::internal::BoolExprImpl>;
+template class scoped_refptr<const sandbox::bpf_dsl::internal::ResultExprImpl>;