Update docs for profiling Content Shell on Android.
Update docs for profiling Content Shell on Android.

This needed to be updated to reflect the move from GYP to GN, and to
show the more modern alternative to manually invoking perf, which is
having Catapult do it for you.

BUG=608414

Review-Url: https://codereview.chromium.org/1950383002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#392675}

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