Fullscreen content is sometimes obscured by taskbar (which even covers up the "exit...

Fullscreen content is sometimes obscured by taskbar (which even covers up the "exit fullscreen" button)
https://bugs.webkit.org/show_bug.cgi?id=61569
<rdar://problem/9454315>

Reviewed by Steve Falkenburg.

Normally, when the background fullscreen window is animated in, the Windows taskbar will be hidden, but this doesn't always work for some reason.
Workaround this issue by setting the real fullscreen window to be a topmost window.

* platform/graphics/win/FullScreenController.cpp:
(FullScreenController::Private::fullscreenClientWndProc): Exit fullscreen when m_fullScreenWindow is deactivated.
(FullScreenController::enterFullScreenRepaintCompleted): Make m_fullScreenWindow a topmost window before animating it in to ensure the taskbar is hidden.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@87442 268f45cc-cd09-0410-ab3c-d52691b4dbfc