Absolute path fixes for gn.

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 d398366e69c6f8e27536d954d2167ec32ebb0b42..73cab568f174bdf3020c058907d0a52648c7926b 100644
--- a/tools/gn/filesystem_utils_unittest.cc
+++ b/tools/gn/filesystem_utils_unittest.cc
@@ -505,6 +505,14 @@ TEST(FilesystemUtils, GetOutDirForSourceDir) {
   EXPECT_EQ("two/obj/foo/bar/",
             GetOutputDirForSourceDirAsOutputFile(
                 &other_settings, SourceDir("//foo/bar/")).value());
+
+  // Absolute source path
+  EXPECT_EQ("//out/Debug/obj/ABS_PATH/abs/",
+            GetOutputDirForSourceDir(
+                &default_settings, SourceDir("/abs")).value());
+  EXPECT_EQ("obj/ABS_PATH/abs/",
+            GetOutputDirForSourceDirAsOutputFile(
+                &default_settings, SourceDir("/abs")).value());
 }
 
 TEST(FilesystemUtils, GetGenDirForSourceDir) {