Clip the start position when performing SVG text queries

If a text query straddles multiple line boxes, only the first would be
considered, because |startPosition| would end up being negative.
Instead clip the character range to [0) (the following functions will
clip against the end position) and only reject ranges that are empty i.e.
that appear before the current fragment (logically.)

BUG=353462

Review URL: https://codereview.chromium.org/206663003

git-svn-id: svn://svn.chromium.org/blink/trunk@169682 bbb929c8-8fbe-4397-9dbb-9b2b20218538