Add runtime dependency extraction for GN.

tools/gn/runtime_deps.h

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef TOOLS_GN_RUNTIME_DEPS_H
+#define TOOLS_GN_RUNTIME_DEPS_H
+
+#include <utility>
+#include <vector>
+
+class Builder;
+class Err;
+class OutputFile;
+class Target;
+
+extern const char kRuntimeDeps_Help[];
+
+// Computes the runtime dependencies of the given target. The result is a list
+// of pairs listing the runtime dependency and the target that the runtime
+// dependency is from (for blaming).
+std::vector<std::pair<OutputFile, const Target*>> ComputeRuntimeDeps(
+    const Target* target);
+
+// Writes all runtime deps files requested on the command line, or does nothing
+// if no files were specified.
+bool WriteRuntimeDepsFilesIfNecessary(const Builder& builder, Err* err);
+
+#endif  // TOOLS_GN_RUNTIME_DEPS_H