wm: Snap to maximized when interactively resizing browser.

This makes the interactive resize triggered by dragging a
browser window's border snap to maximized mode if the user
drags 50 pixels or more beyond the maximum width permitted
for unmaximized windows.

BUG=chromium-os:17160
TEST=manual; also added a simple test

Change-Id: I0514bfecfc6ec913f00fe56111e14c6d1ea826bb
Reviewed-on: http://gerrit.chromium.org/gerrit/4965
Tested-by: David Reveman <reveman@chromium.org>
Reviewed-by: David Reveman <reveman@chromium.org>
Tested-by: Daniel Erat <derat@chromium.org>