Introduce PositionIteratorTest in webkit_unit_tests

This patch introduces |PositionIteratorTest| to record current behavior to catch
regression for improve code health.

This patch is also for preparation of patch[1].

Note: |PositionIteratorTest| needs to have more test cases. This patch
introduces test cases for patch[1].

[1] http://crrev.com/2720593005: Make PositionIterator to skip contents of
INPUT/SELECT/TEXTAREA

BUG=695317
TEST=n/a; no behavior changes

Review-Url: https://codereview.chromium.org/2725803002
Cr-Commit-Position: refs/heads/master@{#453893}
4 files changed