Fix gn bootstrap

R=dpranke

Change-Id: I9f93686d2bab2d3625363142bb05349817e1c5b5
Reviewed-on: https://chromium-review.googlesource.com/982250Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546160}

tools/gn/bootstrap/bootstrap.py

@@ -486,6 +486,7 @@ def write_gn_ninja(path, root_gen_dir, options):
       'base/message_loop/message_pump.cc',
       'base/message_loop/message_pump_default.cc',
       'base/metrics/bucket_ranges.cc',
+      'base/metrics/dummy_histogram.cc',
       'base/metrics/field_trial.cc',
       'base/metrics/field_trial_param_associator.cc',
       'base/metrics/field_trial_params.cc',