Add null check for frame when creating WebCore::Performance.

This regression was introduced in r168256. The previous behavior
depended on whether Frame had already been garbage collected after being
removed from the DOM. If it had not been garbage collected yet, then it
would return a Performance object, but if it had, then it would return
null.

This patch adds the now-necessary check that frame is non-null and makes
the behavior consistent whether or not the underlying Frame object has
already been garbage collected, which makes Firefox's behavior.

BUG=350978
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/203053002

git-svn-id: svn://svn.chromium.org/blink/trunk@169467 bbb929c8-8fbe-4397-9dbb-9b2b20218538