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Use StartupTracingController both for --trace-startup (primary use case:
browser startup perf profiling) and --enable-tracing (primary use case:
browser test tracing).

Due to historical reasons both flags are kept, but now their behaviour
is mostly the same. If both are present, --trace-startup takes
precedence.

Differences:
- `trace-startup` finishes after 5 seconds by default, while `enable-tracing`
  continues until the browser is closed. (--trace-startup-duration overrides
  both).
- `trace-startup-file` always treats the path as absolute, while if
  `enable-tracing-output` ends with /, it uses the provided value as the
  directory and generates the base name based on the current runtime
  metadata.

R=skyostil@chromium.org,eseckler@chromium.org
BUG=1157954

Change-Id: Ia76316aa8927b431784e2fb94c8e42e4e2c0b479
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2589834Reviewed-by: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Commit-Queue: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#839992}