Introduce trace events into Position::upstream() and downstream()
Introduce trace events into Position::upstream() and downstream()

This patch introduces trace events into |Position::upsteram()| and
|downstream()| to study slow part during selecting long word.

BUG=477248
TEST=n/a; no behavior changes

Review URL: https://codereview.chromium.org/1284113002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@200395 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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