Introduce a helper function to get GCType in telemetry.
Introduce a helper function to get GCType in telemetry.

This CL is a preparation to update the arguments of Heap::collectGarbage event.
https://codereview.chromium.org/1046033003/

BUG=438074
TEST=./tools/perf/run_tests oilpan --browser=content-shell-release

Review URL: https://codereview.chromium.org/1054463002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#323192}

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