During window capture on Windows 10, if the selected window is minimized,
ShouldUseScreenCapturer() still thinks it's on top and continue to do a
screencapture which is meaningless.
This cl will set |.is_top_window| with false to minimized window,then we
can skip doing any capture to it.
BUG=chromium:568835
Review-Url: https://codereview.webrtc.org/2997493002
Cr-Commit-Position: refs/heads/master@{#19276}