Tentative fix for crash in WebCore::RangeBoundaryPoint::ensureOffsetIsValid
called from InputMethodController::setCompositionFromExistingText.

BUG=404206

Review URL: https://codereview.chromium.org/529143002

git-svn-id: svn://svn.chromium.org/blink/trunk@181399 bbb929c8-8fbe-4397-9dbb-9b2b20218538