Bits necessary to send the reports along with the minidumps.

Bits necessary to send the reports along with the minidumps.
Currently the log file and the minidump are uploaded in two consequent requests,
thus they get different report ids and it's hard to associate them to each
other.
This CL makes the crash uploader send the minidump and the log file together in
a single multipart request, so that they have the same report id and are
accessible from the same landing page.

[Mark Mentovai]

LGTM and landed r963.