[Win] Add a fast profile switcher to the Windows taskbar item.

[Win] Add a fast profile switcher to the Windows taskbar item.

Mac and Linux now have a fast profile switcher in the OS menu bar, so for the
sake of feature parity, the Windows taskbar should have one as well. Since we
can only have 10 items in total in the Jumplist, we are going to run an
experiment and see if the hypothesis that nobody uses the Most Visited items
in the Jumplist is true (and that we can replace them with a list of Profiles)

I've also added a new histogram, WinJumplist.Action to track what got clicked in
the Jumplisy. Because the Jumplist is just a list of shortcuts, this means adding
a command line switch, that gets checked when the chrome process starts up.

Also includes a small refactor of AvatarMenu::GetImageForMenuButton so that it
can be used without a Profile* (also, it was completely unused on CrOS)

Screenshot: https://drive.google.com/folderview?id=0B1B1Up4p2NRMNzVLMVpfSkJTbmc&usp=sharing

BUG=421490
TEST=Start Chrome. Right click on the Windows taskbar item. A list of profiles
should appear above the Tasks list. Clicking on a profile name should either
open a new browser window for the profile, or activate an existing one.

Review URL: https://codereview.chromium.org/660813002

Cr-Commit-Position: refs/heads/master@{#304078}