Don't append log lines while perf tests are running.

Don't append log lines while perf tests are running.
https://bugs.webkit.org/show_bug.cgi?id=86028

Reviewed by Ryosuke Niwa.

In some tests, the extra DOM nodes can cause the test timing to change.
Avoid adding DOM nodes until the test is over. When running in a browser,
we append nodes as we run so the user can have some feedback.

* resources/runner.js:
(PerfTestRunner.log): Store the log lines in an array until the test is finished.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@116984 268f45cc-cd09-0410-ab3c-d52691b4dbfc