Add reviewers for gn, gni files (#4446) #16720

Job	Run time
chromium_linux-x64 / initialize	1m 18s
linux / initialize	1m 21s
chromium_linux-x64 / docker-build-image	3m 34s
linux / docker-build-image	3m 34s
chromium_linux-x64 / x64_devel	24m 20s
chromium_linux-x64 / x64_debug	25m 34s
chromium_linux-x64 / x64_qa	23m 26s
chromium_linux-x64 / x64_gold	29m 17s
linux / x64_devel	28m 19s
linux / x64_debug	26m 26s
linux / x64_qa	25m 57s
linux / x64_gold	29m 36s
	3h 42m 42s

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