Bug 125619 added an implementation of WebCore::findEndWordBoundary() in Source/WebCore/platform/text/mac/TextBoundaries.mm, but it needs a separate implementation for IOS, much like WebCore::findWordBoundary().
<rdar://problem/15800355>
The iOS build failure was worked around in r161795: <http://trac.webkit.org/changeset/161795>
There is an implementation of this function as of r224244