chromium: Update to latest beta and dev channels

Overview of the updated versions:

beta: 50.0.2661.49 -> 51.0.2704.47
dev:  51.0.2693.2  -> 52.0.2729.3

It has been a while since we had a major Chromium update that compiled
and worked without troubles, but version 52 builds and the VM tests are
successful as well:

https://headcounter.org/hydra/eval/320335



Signed-off-by: aszlig <aszlig@redmoonstudios.org>