Don't assert if selectWordAroundCaret is called with no selection.

The header clearly says that false will be returned if the selection is
not possible. We should not ASSERT when something is an expected entry
especially when that entry is then checked and reacted upon.

BUG=None

Review URL: https://codereview.chromium.org/262403002

git-svn-id: svn://svn.chromium.org/blink/trunk@173406 bbb929c8-8fbe-4397-9dbb-9b2b20218538