[Telemetry] Set user agent through Network.setUserAgentOverride instead of --user-agent.

[Telemetry] Set user agent through Network.setUserAgentOverride instead of --user-agent.

This should fix the following user-agent-related issues:
* user agent string being parsed incorrectly by dbus-send on cros
-> * a few sites (Google calendar I think?) don't like this incorrect user agent, and redirect to different versions of the site
* telemetry uses user agent to determine chrome version, but we manually specify user agent, so it returns a fake version
-> * this causes us to incorrectly say that cros doesn't support tab control


BUG=169296, 167979

[hartmanng, 2013-01-18 16:38:25]

Please take a look

[bulach, 2013-01-18 20:44:04]

+peter

lgtm but.. :)

when I originally added the userAgent, I chatted with peter and there was a
reason we decided to go for command line rather than dev tools... I wished my
memory was working today, sorry! :(

peter, do you remember that discussion? iirc, you had a good reason, but I
forgot...

https://codereview.chromium.org/11880034/diff/9005/tools/telemetry/telemetry/...
File tools/telemetry/telemetry/tab_backend.py (right):

https://codereview.chromium.org/11880034/diff/9005/tools/telemetry/telemetry/...
tools/telemetry/telemetry/tab_backend.py:50: if user_agent_string is not None:
nit: s/is not None//

[nduca, 2013-01-22 21:43:44]

what'd you have to do to make user agent override work btw?

[nduca, 2013-01-22 21:46:03]

https://codereview.chromium.org/11880034/diff/9005/tools/telemetry/telemetry/...
File tools/telemetry/telemetry/tab_backend.py (right):

https://codereview.chromium.org/11880034/diff/9005/tools/telemetry/telemetry/...
tools/telemetry/telemetry/tab_backend.py:41: def _Connect(self):
hmmm... what if connect happens after we've navigated to the page of interest?

The more I think about this, the more we should have devtools give back the
chrome rev directly and continue to use the command line. There seem to be too
many ways for this to go wrong to me to be truly robust.