Add missing file for systrace.

(cherry picked from commit 670892292899fcac00e5fd572f0d9d96acd8847e)

Change-Id: I2d823e6905e66d1ab6c9783cceb0657e904ffca7
diff --git a/build/tools.atree b/build/tools.atree
index a6aa99f..66281ba 100644
--- a/build/tools.atree
+++ b/build/tools.atree
@@ -163,6 +163,7 @@
 
 # systrace
 external/chromium-trace/systrace.py    tools/systrace/systrace.py
+external/chromium-trace/config.py      tools/systrace/config.py
 external/chromium-trace/script.js      tools/systrace/script.js
 external/chromium-trace/style.css      tools/systrace/style.css
 external/chromium-trace/LICENSE        tools/systrace/LICENSE