Minor cleanup in Page::unmarkAllTextMatches.
Minor cleanup in Page::unmarkAllTextMatches.

This is the only core caller of traversePreviousWithWrap(), and it's
never used anymore. Just remove it, since the traverse helpers are
moving into web.

BUG=none

Review URL: https://codereview.chromium.org/1460863005

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#360795}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk