Re-format PositionIterator.cpp to be PRESUBMIT friendly

This patch re-formats "PositionIterator.cpp" to make it PRESUBMIT friendly and
using early-return style for improving code health.

This patch is a preparation of [1]. When I'm working on [1], PRESUBMIT detects
if-return-else pattern.

[1] http://crrev.com/2720593005: Make PositionIterator to handle INPUT, SELECT
TEXTAREA in flat tree correctly.

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

Review-Url: https://codereview.chromium.org/2728493002
Cr-Commit-Position: refs/heads/master@{#453819}
1 file changed