Use a gin::ContextHolder to hold the main blink v8::Context

BUG=321631
R=marja@chromium.org

Review URL: https://codereview.chromium.org/110223002

git-svn-id: svn://svn.chromium.org/blink/trunk@163531 bbb929c8-8fbe-4397-9dbb-9b2b20218538