[DevTools] Mark Tracing.start and Tracing.end as async.

Handlers for these methods are async on the browser side,
so they should be marked "async" for proper code generation.

Drive-by: remove "frontend" from handlers property, as it's not used anywhere.

BUG=none

Review URL: https://codereview.chromium.org/626193004

git-svn-id: svn://svn.chromium.org/blink/trunk@183269 bbb929c8-8fbe-4397-9dbb-9b2b20218538