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
Cleaning up link highlighting FIXMEs

Emergency fix from https://codereview.chromium.org/24376003/ left behind
numerous FIXMEs. This CL cleans up the easy fixes.

BUG=303926

Review URL: https://codereview.chromium.org/25938003

git-svn-id: svn://svn.chromium.org/blink/trunk@159043 bbb929c8-8fbe-4397-9dbb-9b2b20218538