Pass only main thread events into didStopRecordingTraceEvents

Otherwise we will build event stacks comprised of events from different threads.

BUG=361045
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/311783003

git-svn-id: svn://svn.chromium.org/blink/trunk@175339 bbb929c8-8fbe-4397-9dbb-9b2b20218538