More BrowserFileLoader refactoring #771

Job	Run time
Test Browser on macOS (safari, disabled)	1m 10s
Test Build	4m 36s
Test Browser on macOS (safari, main)	1m 55s
Test Browser on Linux (firefox, default)	1m 31s
Test Browser on macOS (safari, default)	2m 0s
Test Shell (spidermonkey, disabled)	2m 23s
Test Browser on Linux (firefox, disabled)	1m 20s
Test Browser on Linux (chrome, disabled)	1m 3s
Test Browser on Linux (firefox, main)	54s
Test Shell (jsc, default)	3m 4s
Test Browser on Linux (chrome, main)	32s
Test Shell (v8, disabled)	1m 35s
Test Browser on Linux (chrome, default)	1m 1s
Test Shell (v8, default)	2m 46s
Test Shell (jsc, disabled)	1m 50s
Test Shell (v8, main)	31s
Test Shell (spidermonkey, default)	5m 19s
Test Shell (jsc, main)	1m 6s
Test Shell (spidermonkey, main)	50s
	35m 26s

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