According to the Chrome source code (https://chromium.googlesource.com/chromium/src/base/+/master/base_switches.cc#120):

The /dev/shm partition is too small in certain VM environments, causing Chrome
to fail or crash (see http://crbug.com/715363). Use this flag to work-around
this issue (a temporary directory will always be used to create anonymous
shared memory files).

Addresses gitlab-org/gitlab-ee#4252 but doesn't appear to cure it completely.