GN: Change gen command syntax, support relative dirs.

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 152851d971cc4a1e167254eae66ae38a76f9842b..ba0863563a8aa2492fd658ec0ac67e49c89f2d45 100644
--- a/tools/gn/filesystem_utils.h
+++ b/tools/gn/filesystem_utils.h
@@ -145,6 +145,17 @@ std::string RebaseSourceAbsolutePath(const std::string& input,
 // a "." so this remains valid.
 std::string DirectoryWithNoLastSlash(const SourceDir& dir);
 
+// Returns the "best" SourceDir representing the given path. If it's inside the
+// given source_root, a source-relative directory will be returned (e.g.
+// "//foo/bar.cc". If it's outside of the source root, a system-absolute
+// directory will be returned.
+SourceDir SourceDirForPath(const base::FilePath& source_root,
+                           const base::FilePath& path);
+
+// Like SourceDirForPath but returns the SourceDir representing the current
+// directory.
+SourceDir SourceDirForCurrentDirectory(const base::FilePath& source_root);
+
 // -----------------------------------------------------------------------------
 
 // These functions return the various flavors of output and gen directories.