Fix cursor getting invisible after moving a window.

The platform cursor value was not correctly set when cursor was unlocked.
This CL fixes it and adds unittests.

BUG=150904


Review URL: https://chromiumcodereview.appspot.com/10965020

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@157924 0039d316-1c4b-4281-b951-d872f2087c98

• ash/ash.gyp[diff]
• ash/test/cursor_manager_test_api.cc[Added - diff]
• ash/test/cursor_manager_test_api.h[Added - diff]
• ash/wm/cursor_manager.cc[diff]
• ash/wm/cursor_manager.h[diff]
• ash/wm/cursor_manager_unittest.cc[Added - diff]