Move HostWindow.h from core/platform to platform in preparation for moving more parts of core/platform/Widget*|Scroll*

BUG=297477

Review URL: https://codereview.chromium.org/26296002

git-svn-id: svn://svn.chromium.org/blink/trunk@159042 bbb929c8-8fbe-4397-9dbb-9b2b20218538