Reland: Improve Benchmark constructor and instance variables #255 (#263) #816

Job	Run time
Test Browser on macOS (safari, main)	53s
Test Browser on Linux (firefox, disabled)	1m 30s
Test Browser on macOS (safari, disabled)	1m 28s
Test Shell (v8, main)	1m 8s
Test Browser on macOS (safari, default)	1m 38s
Test Build	26s
Test Browser on Linux (chrome, default)	1m 11s
Test Shell (jsc, main)	47s
Test Browser on Linux (firefox, default)	2m 0s
Test Shell (jsc, default)	2m 59s
Test Browser on Linux (chrome, disabled)	1m 0s
Test Shell (jsc, disabled)	1m 46s
Test Shell (spidermonkey, default)	5m 10s
Test Shell (v8, default)	2m 42s
Test Shell (spidermonkey, disabled)	1m 49s
Test Shell (v8, disabled)	1m 30s
Test Shell (spidermonkey, main)	48s
Test Browser on Linux (chrome, main)	1m 5s
Test Browser on Linux (firefox, main)	48s
	30m 38s

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