DevTools: Make InspectorTest.runAfterPendingDispatches wait for all targets.

Otherwise, e.g. expandConsoleMessages won't wait for remote objects from workers.

R=sergeyv, vsevik

Review URL: https://codereview.chromium.org/663153002

git-svn-id: svn://svn.chromium.org/blink/trunk@184072 bbb929c8-8fbe-4397-9dbb-9b2b20218538