Fix a binary size regression

This patch changes Jumbo build to use only in debug mode.

tools/setup.py

@@ -104,7 +104,9 @@ def generate_gn_args(mode):
     if mode == "release":
         out += ["is_official_build=true"]
     elif mode == "debug":
-        pass
+        # Enable Jumbo build by default in debug mode for faster build.
+        # https://chromium.googlesource.com/chromium/src/+/master/docs/jumbo.md
+        out += ["use_jumbo_build=true"]
     else:
         print "Bad mode {}. Use 'release' or 'debug' (default)" % mode
         sys.exit(1)