Reland "Set "dev" channel for all lacros builds."

This is a reland of 8d4df26b

Original change's description:
> Set "dev" channel for all lacros builds.
>
> Linux-chrome (of which lacros is a variant) has its channel set as a
> runtime environment variable by the launcher script. For Lacros, we must
> set the environment variable from ash. For now, we always set "dev" for
> all builds of Lacros.
>
> Change-Id: I840286b6fd5772701c3f6d927986405697b82097
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2570448
> Commit-Queue: Erik Chen <erikchen@chromium.org>
> Reviewed-by: James Cook <jamescook@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#833460}

Change-Id: I1d42534b8e289e1df63286658d9c7601357e82dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2617099Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841595}

chrome/browser/chromeos/crosapi/browser_manager.cc

@@ -307,6 +307,9 @@ void BrowserManager::StartWithLogFile(base::ScopedFD logfd) {
   options.environment["GOOGLE_DEFAULT_CLIENT_SECRET"] =
       google_apis::GetOAuth2ClientSecret(google_apis::CLIENT_MAIN);
 
+  // This sets the channel for Lacros.
+  options.environment["CHROME_VERSION_EXTRA"] = "dev";
+
   options.kill_on_parent_death = true;
 
   // Paths are UTF-8 safe on Chrome OS.