Internal IE driver API change GetWindowHandle to GetContentWindowHandle

Changing the base DocumentHost class to use GetContentWindowHandle as the
method name for the method to get the content window handle.
Internal API addition for IE driver to get browser window handle

This commit introduces an API for getting the browser window handle. In
IE6, this would be the top-level window handle. In IE7 and above, this
will be the handle of the tab window. This is separate from the window
which renders the content, which has a window class of
'InternetExplorer_Server'. There is a need to locate both window handles
distinctly in the internal operation of the IE driver.
Fixing passing per-process proxy settings to new windows

Fixes issue #8112.