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Add flag to allow generate_buildbot_json.py script to accept multiple gn isolate map files.

Bug: 1080853
Change-Id: I756cbce16923839ba1ca8fc00f8d67a8df04edbc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2504210
Commit-Queue: Chong Gu <chonggu@google.com>
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/master@{#821799}