This knob was added back in 2014 in commit fb6df963376c ("chromium: Add
recipe for version 37.0.2062.0"). Multithreaded rasterization was under
development back then, but this hasn't been the case since M39 and M40 (see
https://crbug.com/169282 and https://crbug.com/413479).
Given multithreaded rasterization was enabled by default in 2015, the
--enable-impl-side-painting was removed around that time and that this knob
was off by default, it should be safe to remove it altogether.
This knob was added back in 2014 in commit fb6df963376c ("chromium: Add recipe for version 37.0.2062.0"). Multithreaded rasterization was under development back then, but this hasn't been the case since M39 and M40 (see https://crbug.com/169282 and https://crbug.com/413479).
Given multithreaded rasterization was enabled by default in 2015, the --enable-impl-side-painting was removed around that time and that this knob was off by default, it should be safe to remove it altogether.