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This is a reland of 0823dbae

Original change's description:
> [tracing] Align --trace-startup and --enable-tracing.
>
> 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/+/2589834
> Reviewed-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}

TBR=skyostil@chromium.org,eseckler@chromium.org,nasko@chromium.org

Bug: 1157954
Change-Id: Ic16992026beebddea39c8a6b247b4301512057e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2609139
Commit-Queue: Alexander Timin <altimin@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840015}