Improve the find word boundary performance

In endWordBoundary case, finding start boundary in findWordBoundary is unnecessary.
So, skip it to improve the find word boundary performance.

Merge from WebKit
https://bugs.webkit.org/show_bug.cgi?id=125619

BUG= 371249

Review URL: https://codereview.chromium.org/268363016

git-svn-id: svn://svn.chromium.org/blink/trunk@178621 bbb929c8-8fbe-4397-9dbb-9b2b20218538