Implement `gn analyze`.

tools/gn/filesystem_utils.h

Index: tools/gn/filesystem_utils.h
diff --git a/tools/gn/filesystem_utils.h b/tools/gn/filesystem_utils.h
index f53f24a3eb154e57f5a505136c565906178c187d..3e3c76d8324da50b1da207e82108d9a9d7e2d6d6 100644
--- a/tools/gn/filesystem_utils.h
+++ b/tools/gn/filesystem_utils.h
@@ -175,6 +175,11 @@ bool WriteFileIfChanged(const base::FilePath& file_path,
                         const std::string& data,
                         Err* err);
 
+// Writes given stream contents to the given file. Returns true if data was
+// successfully written, false otherwise. |err| is set on error if not nullptr.
+bool WriteFile(const base::FilePath& file_path, const std::string& data,
+               Err* err);
+
 // -----------------------------------------------------------------------------
 
 enum class BuildDirType {