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This adds the --urls flag which accepts a space-separated list of urls
to distill.

This adds a pretty straightforward approach to having multiple of these
requests happening at the same time.

Once all requests are finished, they will be printed (in the order that
they appeared in --urls). If printing binary, each serialized protobuf
will be preceded by its size (as a varint32).

TBR=ben@

Review URL: https://codereview.chromium.org/276553002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@276819 0039d316-1c4b-4281-b951-d872f2087c98