Wait a major task after triggering GC to measure memory

[andrewiggins]

window.gc synchronously triggers one Major GC. However that MajorGC asynchronously triggers additional MajorGCs until the usedJSHeapSizeBefore and usedJSHeapSizeAfter are the same. Here, we'll wait a moment (i.e. a major task using postMessage) for some (hopefully all) additional GCs to finish before measuring the memory.

[coveralls]

Coverage: 99.699%. Remained the same when pulling 1345125 on improve-bench-memory-measuring into e97da39 on master.

[github-actions]

Size Change: 0 B

Total Size: 54.3 kB

ℹ️ View Unchanged

Filename	Size	Change
compat/dist/compat.js	3.91 kB	0 B
compat/dist/compat.module.js	3.84 kB	0 B
compat/dist/compat.umd.js	3.98 kB	0 B
debug/dist/debug.js	3 kB	0 B
debug/dist/debug.module.js	3.01 kB	0 B
debug/dist/debug.umd.js	3.08 kB	0 B
devtools/dist/devtools.js	232 B	0 B
devtools/dist/devtools.module.js	240 B	0 B
devtools/dist/devtools.umd.js	315 B	0 B
dist/preact.js	4.21 kB	0 B
dist/preact.min.js	4.24 kB	0 B
dist/preact.min.module.js	4.24 kB	0 B
dist/preact.min.umd.js	4.26 kB	0 B
dist/preact.module.js	4.23 kB	0 B
dist/preact.umd.js	4.27 kB	0 B
hooks/dist/hooks.js	1.53 kB	0 B
hooks/dist/hooks.module.js	1.56 kB	0 B
hooks/dist/hooks.umd.js	1.61 kB	0 B
jsx-runtime/dist/jsxRuntime.js	360 B	0 B
jsx-runtime/dist/jsxRuntime.module.js	326 B	0 B
jsx-runtime/dist/jsxRuntime.umd.js	441 B	0 B
test-utils/dist/testUtils.js	442 B	0 B
test-utils/dist/testUtils.module.js	444 B	0 B
test-utils/dist/testUtils.umd.js	526 B	0 B

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[andrewiggins]

FYI, microtask is too quick and setTimeout can add extra wait time so I went with post message as it seems to immediately fire after GC has stabilized, judging from the browser traces I looked at.