Get redundant using statements from VisibuleUnits.cpp
Get redundant using statements from VisibuleUnits.cpp

This patch gets rid of redundant |using| statements which define |PositionType|
alias in "VisibleUnits.cpp" to simplify source code for improving readability.

This patch is a follow up patch of http://crrev.com/1261573004, which asked to
do so.

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

Review URL: https://codereview.chromium.org/1263943003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@199796 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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