Make BQWrite accept a write disposition

Bug:615218
Change-Id: Iaf69bcb852ee28bb0f691df71edcab01b74ac06f
Reviewed-on: https://chromium-review.googlesource.com/656083
Commit-Queue: Katie Thomas <katthomas@google.com>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>

Cr-Mirrored-From: https://chromium.googlesource.com/infra/infra
Cr-Mirrored-Commit: c36018d35225ce3c0c6cc00c89e5c01e7d73effc
diff --git a/common/chops_beam.py b/common/chops_beam.py
index 50687fd..20ffad5 100644
--- a/common/chops_beam.py
+++ b/common/chops_beam.py
@@ -37,8 +37,8 @@
      beam.io.BigQuerySink will automatically add unique insert ids to rows,
      which BigQuery uses to prevent duplicate inserts.
   """
-  def __init__(self, table, dataset='aggregated'):
-    write_disposition = beam.io.BigQueryDisposition.WRITE_TRUNCATE
+  def __init__(self, table, dataset='aggregated',
+               write_disposition=beam.io.BigQueryDisposition.WRITE_TRUNCATE):
     sink = beam.io.BigQuerySink(table, dataset, project='chrome-infra-events',
                                 write_disposition=write_disposition)
     super(BQWrite, self).__init__(sink)