Windows: Top-level bubbles no longer appear on taskbar by default.

On Windows, Widget::InitParams::force_show_in_taskbar is now respected,
so it is possible to override the default without having to directly set
HWND properties.

This should not change any behaviour (just a refactor).

Previously, we had a special case where the message center would turn
off showing in taskbar, and app launcher would turn it off for <= Vista.
Now we have just one special case: app launcher turns it on for >= Win7.

BUG=332277

Review URL: https://codereview.chromium.org/127133003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243857 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243857