tracing: Use TRACE_EVENT_NESTABLE_ASYNC_* for NavigationRequest events

The deprecated TRACE_EVENT_ASYNC* macros have been superseded by
TRACE_EVENT_NESTABLE_ASYNC*.

Updating to the new macros is mostly straight-forward, except in cases
where STEP_INTO/PAST macros were used extensively: In those cases, the
steps have been converted into nested sub-events.

For NavigationRequest, we also have to split the StartToCommit event
into its own separate event track, so that it doesn't get merged onto
the same track as the remaining events in NavigationRequest. We
accomplish this with a scoped event id.

Also removes duplicate async start/end events in this class.

Bug: 1038710
Change-Id: I44fe91e24291751ea06ab447309277d7eef26529
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2075338
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Auto-Submit: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745523}

content/browser/frame_host/navigation_request.h

@@ -844,10 +844,6 @@ class CONTENT_EXPORT NavigationRequest
     return std::move(modified_request_headers_);
   }
 
-  // Helper functions to trace the start and end of |navigation_handle_|.
-  void TraceNavigationStart();
-  void TraceNavigationEnd();
-
   // Returns true if the contents of |common_params_| requires
   // |source_site_instance_| to be set. This is used to ensure that data:
   // URLs with valid initiator origins always have |source_site_instance_| set