traffic annotation: Use checked in version of gn

Bug: 731558
Change-Id: I4d3eaf86139e140df0c465edfdc84756a490fa38
Reviewed-on: https://chromium-review.googlesource.com/896954Reviewed-by: Ramin Halavati <rhalavati@chromium.org>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534127}

tools/traffic_annotation/auditor/traffic_annotation_auditor.cc

@@ -491,8 +491,13 @@ bool TrafficAnnotationAuditor::CheckIfCallCanBeUnannotated(
     // Check if the file including this function is part of Chrome build.
     const base::CommandLine::CharType* args[] = {
 #if defined(OS_WIN)
-      FILE_PATH_LITERAL("gn.bat"),
+      FILE_PATH_LITERAL("buildtools/win/gn.exe"),
+#elif defined(OS_MACOSX)
+      FILE_PATH_LITERAL("buildtools/mac/gn"),
+#elif defined(OS_LINUX)
+      FILE_PATH_LITERAL("buildtools/linux64/gn"),
 #else
+      // Fallback to using PATH to find gn.
       FILE_PATH_LITERAL("gn"),
 #endif
       FILE_PATH_LITERAL("refs"),