Factoring duplicate code from platform-specific LoginHandlers into a base ...

Factoring duplicate code from platform-specific LoginHandlers into a base 
LoginHandler class. As pointed out by erg, it would be cleaner to use a 
separate controller class, but this simple refactor should make that easier. 

Originally submitted as 41739, however that patch had to be reverted because
I had accidentally changed an if (!...) { NOTREACHED(); } into a DCHECK(...) in
the LoginHandler ctor. That is fixed now.

BUG=14909
TEST=ui_tests --gtest_filter=LoginPromptTest*


Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=42368

[tonyg, 2010-03-17 19:33:39]

I made a totally braindead mistake before by guarding a call that should always
happen within a DCHECK. That's why this worked in Debug mode locally and on the
trybots. I've verified locally that everything works in Release mode on mac and
linux.

[Elliot Glaysher, 2010-03-17 21:06:04]

LGTM, but you're going to have to get someone else to commit it. I'm out sick.

[tonyg, 2010-03-19 16:28:53]

On 2010/03/17 21:06:04, Elliot Glaysher wrote:
> LGTM, but you're going to have to get someone else to commit it. I'm out sick.

Hi Eric, Your name showed up on blame for login_prompt.cc. Would you mind taking
a brief look at this and landing it if you are okay with it? Thanks, Tony