Always set NSTexturedBackgroundWindowMask on app windows.
This is consistent with FramedBrowserWindow.
This also skips drawing the frame of frameless windows.
On OSX 10.7, if NSTexturedBackgroundWindowMask is set, NSGrayFrame is
used to draw the frame. If not, NSThemeFrame is used. Since we swizzle
drawing methods on NSGrayFrame on 10.7, the app windows should all use
NSGrayFrame.
BUG=378581
Review URL: https://codereview.chromium.org/
307843002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@273781 0039d316-1c4b-4281-b951-d872f2087c98