Fix typos in BUILD.gn.

TBR=weili@chromium.org

Review URL: https://codereview.chromium.org/1419763005 .

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index 70fb380..a9d58a1 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -153,7 +153,7 @@
     "testing/fx_string_testhelpers.h",
     "testing/test_support.cpp",
     "testing/test_support.h",
-    'testing/utils/path_service.cpp',
+    "testing/utils/path_service.cpp",
   ]
   deps = [
     "//testing/gmock",