Improve tracing of the main frame pipeline

This patch improves the tracing of the main frame pipeline, ensuring
that there is a single flow connecting all events from
SendBeginMainFrame to MainFrame.Draw.

This patch also removes duplication of the events (e.g. now the flow is
connected to ProxyMain::BeginMainFrame slice instead of a separate
MainFrame.BeginMainFrameOnMain.

This patch also makes the relevant arguments typed, upgrades the flow id
to the modern perfetto::Flow and makes the main frame trace ids
type-safe.

Change-Id: I5208c2c73885b2545038e073c64fb31d4b4130e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5836797
Reviewed-by: Jonathan Ross <jonross@chromium.org>
Commit-Queue: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1352036}