Refactor unitTable access

Following the review of https://codereview.chromium.org/209443002/ , I've changed the unitTable access method to be a static CSSPrimitiveValue::fromName method.

BUG=

Review URL: https://codereview.chromium.org/209443003

git-svn-id: svn://svn.chromium.org/blink/trunk@169802 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/core/css/CSSPrimitiveValue.cpp

@@ -140,10 +140,10 @@ CSSPrimitiveValue::UnitTable createUnitTable()
     return table;
 }
 
-CSSPrimitiveValue::UnitTable& CSSPrimitiveValue::getUnitTable()
+CSSPrimitiveValue::UnitTypes CSSPrimitiveValue::fromName(const String& unit)
 {
     DEFINE_STATIC_LOCAL(UnitTable, unitTable, (createUnitTable()));
-    return unitTable;
+    return unitTable.get(unit.lower());
 }
 
 CSSPrimitiveValue::UnitCategory CSSPrimitiveValue::unitCategory(CSSPrimitiveValue::UnitTypes type)

third_party/WebKit/Source/core/css/CSSPrimitiveValue.h

@@ -150,7 +150,7 @@ public:
     static UnitCategory unitCategory(CSSPrimitiveValue::UnitTypes);
 
     typedef HashMap<String, CSSPrimitiveValue::UnitTypes> UnitTable;
-    static UnitTable& getUnitTable();
+    static UnitTypes fromName(const String& unit);
 
     bool isAngle() const
     {

third_party/WebKit/Source/core/css/parser/MediaQueryToken.cpp

@@ -49,7 +49,7 @@ void MediaQueryToken::convertToDimensionWithUnit(String unit)
 {
     ASSERT(m_type == NumberToken);
     m_type = DimensionToken;
-    m_unit = CSSPrimitiveValue::getUnitTable().get(unit.lower());
+    m_unit = CSSPrimitiveValue::fromName(unit);
 }
 
 void MediaQueryToken::convertToPercentage()