Improve Benchmark constructor and instance variables #783

Job	Run time
Test Build	25s
Test Browser on Linux (chrome, main)	35s
Test Browser on Linux (firefox, main)	42s
Test Browser on Linux (firefox, default)	1m 29s
Test Browser on macOS (safari, main)	48s
Test Shell (spidermonkey, disabled)	1m 52s
Test Browser on Linux (chrome, disabled)	1m 4s
Test Shell (v8, disabled)	1m 36s
Test Browser on Linux (firefox, disabled)	1m 18s
Test Browser on macOS (safari, default)	1m 12s
Test Shell (jsc, default)	3m 4s
Test Browser on Linux (chrome, default)	1m 10s
Test Shell (spidermonkey, main)	47s
Test Browser on macOS (safari, disabled)	1m 23s
Test Shell (v8, main)	40s
Test Shell (v8, default)	2m 41s
Test Shell (jsc, main)	45s
Test Shell (jsc, disabled)	1m 46s
Test Shell (spidermonkey, default)	5m 14s
	28m 31s

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