DevTools: process trace events from background threads by a timed task.

We used to process pending background events upon BeginFrame, which
was a hack and does not work if main thread frames are not fired.
We should post a main thread task upon adding new events instead.

BUG=305863
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/27524002

git-svn-id: svn://svn.chromium.org/blink/trunk@160160 bbb929c8-8fbe-4397-9dbb-9b2b20218538