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Use multiprocess.Pool to spawn CPU# / 2 processes and accelerate
generate_json_dependency_graph.py when possible.

Based on a single run:

Before:
real	5m38.769s
user	29m9.770s
sys	1m38.073s

After:
real	1m11.504s
user	23m35.792s
sys	1m5.959s

Bug: 1081840
Change-Id: Ib5d1e24c37d1fadeae5df0536c7fd1b20e7a1b60
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2285379
Commit-Queue: Henrique Nakashima <hnakashima@chromium.org>
Reviewed-by: Peter Wen <wnwen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#786438}