Get rid of redundant using statements in VisibleUnit.cpp

This patch gets rid of redundant |using| statements in "VisibleUnit.cpp"
for improving code health.

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

Review URL: https://codereview.chromium.org/1305973006

git-svn-id: svn://svn.chromium.org/blink/trunk@201709 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/core/editing/VisibleUnits.cpp

@@ -61,9 +61,6 @@
 
 namespace blink {
 
-using namespace HTMLNames;
-using namespace WTF::Unicode;
-
 template <typename PositionType>
 static PositionType canonicalizeCandidate(const PositionType& candidate)
 {