Fix |RunMicrotasks()| leaking reference to the last context being run on.

RunMicrotasks() executes pending tasks and swaps the old array with a new array. However, the new array contains the reference to the current context as its creation context. This prevents the context from gc-ed until RunMicrotasks() is executed in the different context.

BUG=crbug.com/367016
LOG=y
R=rafaelw@chromium.org, rossberg@chromium.org

Review URL: https://codereview.chromium.org/250883002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21111 ce2b1a6d-e550-0410-aec6-3dcde31c8c00