Only disable DWM NC painting if the windows have custom regions.

This changes the conditions under which DWM NC painting is enabled or disabled. Now, DWM NC painting is only disabled when windows have custom regions, e.g. if they use the chrome window shaping API. This only effects windows which set their region using custom_window_region, not those shaped via GetWindowMask.

The change also sets the NC painting to either DISABLED or USEWINDOWSTYLE, as ENABLED is not appropriate for windows like modal dialogs which, despite having a custom frame do not need the DWM NC rendering.
BUG=329360

Review URL: https://codereview.chromium.org/129833006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246528 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246528