TL;DR: right after launch, Chrome forks off a helper process which is then responsible for forking off child processes, so that all child processes use the same process image as the main Chrome process. It also opens all files it will need right after launch and just re-uses the same file descriptors, so it always keeps references to the old files. This is complicated and brittle and a source of bugs, but when everything works and nobody makes a mistake, it means the old Chrome can keep running without problems even after all its files have been replaced with those from a new Chrome.
At least that's how it worked in 2011 according to Evan Martin.
TL;DR: right after launch, Chrome forks off a helper process which is then responsible for forking off child processes, so that all child processes use the same process image as the main Chrome process. It also opens all files it will need right after launch and just re-uses the same file descriptors, so it always keeps references to the old files. This is complicated and brittle and a source of bugs, but when everything works and nobody makes a mistake, it means the old Chrome can keep running without problems even after all its files have been replaced with those from a new Chrome.
At least that's how it worked in 2011 according to Evan Martin.