GN: Make file/dir resolving return errors.

tools/gn/filesystem_utils_unittest.cc

Index: tools/gn/filesystem_utils_unittest.cc
diff --git a/tools/gn/filesystem_utils_unittest.cc b/tools/gn/filesystem_utils_unittest.cc
index 9f874ebd4f58a8840d5b7bc5e67b5dced66b31e5..14bc9515307e3767a108ff2f358a7789b68eb7a5 100644
--- a/tools/gn/filesystem_utils_unittest.cc
+++ b/tools/gn/filesystem_utils_unittest.cc
@@ -236,6 +236,11 @@ TEST(FilesystemUtils, NormalizePath) {
   input = "foo\\..\\..\\bar";
   NormalizePath(&input);
   EXPECT_EQ("../bar", input);
+
+  // Trailing slashes should get preserved.
+  input = "//foo/bar/";
+  NormalizePath(&input);
+  EXPECT_EQ("//foo/bar/", input);
 }
 
 TEST(FilesystemUtils, RebasePath) {