Add --force-gc support #763

Job	Run time
Test Browser on macOS (safari, main)	1m 6s
Test Browser on macOS (safari, default)	2m 4s
Test Build	24s
Test Browser on Linux (firefox, disabled)	1m 15s
Test Browser on macOS (safari, disabled)	1m 38s
Test Browser on Linux (chrome, main)	1m 16s
Test Browser on Linux (firefox, main)	44s
Test Browser on Linux (firefox, default)	1m 33s
Test Browser on Linux (chrome, disabled)	1m 7s
Test Shell (spidermonkey, default)	5m 16s
Test Shell (jsc, disabled)	1m 45s
Test Shell (v8, disabled)	1m 34s
Test Shell (jsc, default)	3m 16s
Test Browser on Linux (chrome, default)	1m 5s
Test Shell (spidermonkey, main)	51s
Test Shell (spidermonkey, disabled)	2m 10s
Test Shell (v8, main)	41s
Test Shell (v8, default)	2m 44s
Test Shell (jsc, main)	43s
	31m 12s

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