Center and change the aspect ratio for the experimental app list. [Win]

This CL adds the enable-experimental-app-list flag which currently makes the
app list launch to the center of the screen and changes the app list to display
a 6 by 3 grid.

BUG=

Review URL: https://codereview.chromium.org/162753002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251642 0039d316-1c4b-4281-b951-d872f2087c98
Move Windows-specific code into NativeAppWindowViewsWin

NativeAppWindowViews has Windows-specific code in #ifdefs. Move this
into derived class NativeAppWindowViewsWin.

BUG=None

Review URL: https://codereview.chromium.org/164983002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251643 0039d316-1c4b-4281-b951-d872f2087c98