Close and join pool after resolving tombstones.

Bug: 784527
Change-Id: I6f57b53db6f3f898238efa4617e5d8e0bbbfae36
Reviewed-on: https://chromium-review.googlesource.com/776014Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Zhiling Huang <hzl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517508}

build/android/tombstones.py

@@ -131,6 +131,8 @@ def _ResolveTombstones(jobs, tombstones, tombstone_symbolizer):
     data = pool.map(
         _ResolveTombstone,
         [[tombstone, tombstone_symbolizer] for tombstone in tombstones])
+    pool.close()
+    pool.join()
   resolved_tombstones = []
   for tombstone in data:
     resolved_tombstones.extend(tombstone)