DevTools: switch devtools platform events to new timeline category

This switches trace events emitted from PlatformInstrumentation to use
trace event category consistent with the trace-based timeline.
Drive-by: fix category filter in TraceEventDispatcher.

BUG=361045

Review URL: https://codereview.chromium.org/337143002

git-svn-id: svn://svn.chromium.org/blink/trunk@176413 bbb929c8-8fbe-4397-9dbb-9b2b20218538