Simplify (and optimize) SVGTextQuery::characterNumberAtPositionCallback.
Blink merge: https://codereview.chromium.org/353003002
Pull request: https://github.com/WebKit/WebKit/pull/18392
Committed 268628@main (549366b2482f): <https://commits.webkit.org/268628@main> Reviewed commits have been landed. Closing PR #18392 and removing active labels.
<rdar://problem/116218214>