[Telemetry perf] Only track mean_pixels_approximated & mean_pixels_checkerboarded for smoothness.tough_scrolling_cases
[Telemetry perf] Only track mean_pixels_approximated & mean_pixels_checkerboarded for smoothness.tough_scrolling_cases

[PERF SHERRIF] This may cause alert of dashboard stops outputting
these metrics but they are intentional.

BUG=529331

Review URL: https://codereview.chromium.org/1415763002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#354938}

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