[GN] Don't rewrite files with the same contents

tools/gn/filesystem_utils.h

 // Copyright (c) 2013 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_FILESYSTEM_UTILS_H_
 #define TOOLS_GN_FILESYSTEM_UTILS_H_
 
 #include <stddef.h>
 
 #include <string>
@@ skipping 145 matching lines @@
 // Like SourceDirForPath but returns the SourceDir representing the current
 // directory.
 SourceDir SourceDirForCurrentDirectory(const base::FilePath& source_root);
 
 // Given the label of a toolchain and whether that toolchain is the default
 // toolchain, returns the name of the subdirectory for that toolchain's
 // output. This will be the empty string to indicate that the toolchain outputs
 // go in the root build directory. Otherwise, the result will end in a slash.
 std::string GetOutputSubdirName(const Label& toolchain_label, bool is_default);
 
+// Returns true if the contents of the file and stream given are equal, false
+// otherwise.
+bool ContentsEqual(const base::FilePath& file_path, const std::string& data);
+
+// Writes given stream contents to the given file if it differs from existing
+// file contents. Returns true if new contents was successfully written or
+// existing file contents doesn't need updating, false on write error. |err| is
+// set on write error if not nullptr.
+bool WriteFileIfChanged(const base::FilePath& file_path,
+                        const std::string& data,
+                        Err* err);
+
 // -----------------------------------------------------------------------------
 
 // These functions return the various flavors of output and gen directories.
 SourceDir GetToolchainOutputDir(const Settings* settings);
 SourceDir GetToolchainOutputDir(const BuildSettings* build_settings,
                                 const Label& label, bool is_default);
 
 SourceDir GetToolchainGenDir(const Settings* settings);
 OutputFile GetToolchainGenDirAsOutputFile(const Settings* settings);
 SourceDir GetToolchainGenDir(const BuildSettings* build_settings,
@@ skipping 21 matching lines @@
 
 SourceDir GetTargetOutputDir(const Target* target);
 OutputFile GetTargetOutputDirAsOutputFile(const Target* target);
 SourceDir GetTargetGenDir(const Target* target);
 OutputFile GetTargetGenDirAsOutputFile(const Target* target);
 
 SourceDir GetCurrentOutputDir(const Scope* scope);
 SourceDir GetCurrentGenDir(const Scope* scope);
 
 #endif  // TOOLS_GN_FILESYSTEM_UTILS_H_