Replace explicit Enter/Exit calls with v8::Context::Scope in unit test.

R=abarth@chromium.org,marja@chromium.org,yhirano@chromium.org

Review URL: https://chromiumcodereview.appspot.com/21075006

git-svn-id: svn://svn.chromium.org/blink/trunk@155287 bbb929c8-8fbe-4397-9dbb-9b2b20218538