Add default value for second argument of CSSComputedStyleDeclaration::getPropertyCSSValue()

Add default value for second argument of CSSComputedStyleDeclaration::getPropertyCSSValue()
instead of overloading. This avoids incurring the cost of an extra function call (since the
overload was not inlined) only to provide UpdateLayout as default value for the
EUpdateLayout argument.

R=esprehn@chromium.org, dstockwell@chromium.org

Review URL: https://codereview.chromium.org/286403004

git-svn-id: svn://svn.chromium.org/blink/trunk@174394 bbb929c8-8fbe-4397-9dbb-9b2b20218538