PartitionAlloc is used by Blink extensively, hence CLs which touch its implementation ought to trigger webkit_tests on the trybots.

This does not appear to be happening, e.g., https://codereview.chromium.org/2741853007/ (ps#2, linux_chromium_rel_ng - https://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_rel_ng/builds/407071) did not run layout tests, requiring WebKit/Source/ to be trivially touched to trigger the tests.