Morph the selector widget to a constant padding on the window shapes in overview.

BUG=276502
TEST=Create several differently shaped windows (i.e. some tall, some wide).
Begin alt tabbing between them. When in overview, the selector should morph to
match the selected windows size (plus some padding).

Review URL: https://codereview.chromium.org/32913002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230135 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230135