Upgrade a TextIterator ASSERT to a RELEASE_ASSERT as a defensive measure.

BUG=156930,177197
R=inferno@chromium.org

Review URL: https://codereview.chromium.org/15057010

git-svn-id: svn://svn.chromium.org/blink/trunk@150123 bbb929c8-8fbe-4397-9dbb-9b2b20218538
static inline bool fullyClipsContents(Node* node)
{
    RenderObject* renderer = node->renderer();
    if (!renderer || !renderer->isBox() || !renderer->hasOverflowClip())
        return false;
    return toRenderBox(renderer)->size().isEmpty();
}

static inline bool fullyClipsContents(Node* node)
{
    RenderObject* renderer = node->renderer();
    if (!renderer || !renderer->isBox() || !renderer->hasOverflowClip())
        return false;
    return toRenderBox(renderer)->size().isEmpty();
}
