Bump third_party/imgui/repo from `3c435c0` to `3b8c7d0` #2386

Job	Run time
Build on Darwin (clang, simulated)	55m 58s
Build on Linux (fake, gcc_release, clang, simulated)	53m 2s
Build on Linux (coverage)	13m 4s
Build on Linux (gcc_debug)	1h 5m 50s
Build on Linux (python_lib)	25m 13s
Build on Linux (python lighting-app)	6m 58s
	3h 40m 5s

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