Currently, when specifying '--help' with mksnapshot it will only
print the v8/d8 help message and options and then exit the process.
This means that the usage message from mksnapshot will never be
displayed.

This commit suggests adding an option to SetFlagsFromCommandLine that
can disable this printing and exiting. This allows mksnapshot to display
the usage and print the options after that.

While this works, it does seems a little strange that
SetFlagsFromCommandLine prints the help message and exits the process
but I'm probably missing some background details around this.

Change-Id: I28932adf3478b88b05eed4db70bf74946f8abf2d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2290852Reviewed-by: Dan Elphick <delphick@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68851}