Make previousPositionOf() to handle yet another no previous position case
This patch makes |previousPositionOf()| to handle no previous position case,
which is occurred after visible position canonicalization.
It is time to change |DCEHCK()| to return nothing since we have a test case
for it.
BUG=619131
TEST=run_webkit_unit_tests --gtest_filter=VisibleUnitTests.previousPositionOfNoPreviousPosition
Review-Url: https://codereview.chromium.org/2126743002
Cr-Commit-Position: refs/heads/master@{#403887}
2 files changed
