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This allows working on mojo connections without deploying both binaries
to a device.

* Avoid DCHECK when calling lacros_process_.Pid() on a null process
  handle
* Forward ash-chrome's XDG_RUNTIME_DIR because on linux we can't
  write to /run/chrome
* Use a subdir of the ash-chrome user-data-dir instead of a
  hard-coded path

You can now launch lacros-chrome via the shelf icon if you run
ash-chrome like this:
XDG_RUNTIME_DIR=/tmp/ash_chrome_xdg_runtime ./out/Default/chrome --user-data-dir=/tmp/chrome --enable-wayland-server --no-startup-window --login-manager --enable-features=LacrosSupport --lacros-chrome-path=/w/chrome/src/out/LacrosDesktop

Bug: none
Change-Id: I94a5319174b5952cb6605bf2243821e51ae30ebf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2255032
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#780909}