Add user-agent header to the format spec of EventLogging's varnishncsa instance

User-agent strings often encode sufficiently many bits of information about the user's
setup that they can be used to pinpoint an individual user, especially when
used in conjunction with other seemingly-anonymous datapoints. At the same
time, user-agent strings are indispensable for performance measurements and
front-end error reporting. Historically, the approach of EventLogging was to
leave user-agent logging and processing to developers and analysts. The problem
with this approach is that the anonymization of user-agent fields is not
uniform. When it is done at all, it is often done inconsistently. This often
ends up being a barrier for releasing performance-related datasets that would
greatly benefit from additional scrutiny.

The solution is to centralize the processing of user-agents at the time of data
collection. This way we can be confident that all UAs logged via EventLogging
are adequately anonymized.

The work is being done by the Analytics team (yay!). This is one piece of it.
This patch updates the log format specifier supplied to EventLogging's
varnishncsa instance to include the user-agent header.

Bug: 52295
Change-Id: Ia96008f80e0eee5a96412702b096d0eece7cfba7