Fix gn bootstrap

TBR=dpranke

Change-Id: I72e33bdcd009b62853434507f3096199e921c28d
Reviewed-on: https://chromium-review.googlesource.com/1005446
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#549588}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: e20ec0676c3adbdb4fb37acc17e692cb397f8ed0
diff --git a/tools/gn/bootstrap/bootstrap.py b/tools/gn/bootstrap/bootstrap.py
index 8f32161..e916dad 100755
--- a/tools/gn/bootstrap/bootstrap.py
+++ b/tools/gn/bootstrap/bootstrap.py
@@ -533,6 +533,7 @@
       'base/message_loop/message_loop_task_runner.cc',
       'base/message_loop/message_pump.cc',
       'base/message_loop/message_pump_default.cc',
+      'base/message_loop/watchable_io_message_pump_posix.cc',
       'base/metrics/bucket_ranges.cc',
       'base/metrics/dummy_histogram.cc',
       'base/metrics/field_trial.cc',