tools/gn: Fix for bootstrap.py to keep it working.

It was missing a trace_event source file to keep things compiling.

BUG=None
TEST=./tools/gn/bootstrap/bootstrap.py
TBR=brettw@chromium.org

Review URL: https://codereview.chromium.org/1376133002

Cr-Original-Commit-Position: refs/heads/master@{#351491}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 38275d22a667c2da73c35ab3cd2b8c26f86c9411
1 file changed
tree: 1bcf273eb886954aea34b1c65b8179cc82cee8e9
  1. tools/