Chrome use-mobile-user-agent not working

Running chrome from command line with flag --use-mobile-user-agent does not open the browser in mobile context (user-agent).

chrome --use-mobile-user-agent= true

Note:

passing user-agent option does work, but i feel its not the right way of doing things as chrome offers you this flag to boot in mobile context.

--user-agent= Mozilla/5.0 (iPhone; U; CPU iPhone OS 5_1_1 like Mac OS X; ar) AppleWebKit/534.46.0 (KHTML, like Gecko) CriOS/19.0.1084.60 Mobile/9B206 Safari/7534.48.3

Chromium source code

reading some of the chromium source code, i see the following:

content_switches.cc

define kUseMobileUserAgent from "use-mobile-user-agent" flag: Set when Chromium should use a mobile user agent.

const char kUseMobileUserAgent[] = "use-mobile-user-agent";

shell_content_client.cc

add "Mobile" to product if our variable switch is true/set.

std::string GetShellUserAgent() {
  std::string product = "Chrome/" CONTENT_SHELL_VERSION;
  base::CommandLine* command_line = base::CommandLine::ForCurrentProcess();
  if (command_line->HasSwitch(switches::kUseMobileUserAgent))
    product += " Mobile";
  return BuildUserAgentFromProduct(product);
}

Extra detail (running from selenium)

As an extra detail, i run chrome in using selenium and pass the configurations:

 ...

 "browserName": "chrome",
 "chromeOptions": {
     "args": [
         "--user-agent= Mozilla/5.0 (iPhone; U; CPU iPhone OS 5_1_1 like Mac OS X; ar) AppleWebKit/534.46.0 (KHTML, like Gecko) CriOS/19.0.1084.60 Mobile/9B206 Safari/7534.48.3",
         "--window-size=320,640",
         "--disable-popup-blocking",
         "--incognito",
         "--test-type"
     ]
 },

 ...

The string is built to "Chrome/53.0.2785.116 Mobile" in GetShellUserAgent, then in BuildUserAgentFromProduct, product is not used, and passed on to BuildUserAgentFromOSAndProduct, which is supposed to format a string as such;

"Mozilla/5.0 (%s) AppleWebKit/%d.%d (KHTML, like Gecko) %s Safari/%d.%d"

The product string is inserted into token four, where the fourth replacement token is before "Safari". Therefore "Chrome/53.0.2785.116 Mobile" should be placed there.

With and without the flag, my user agent is the same.

Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/53.0.2785.116 Safari/537.36

So what does this mean, is it broken? Quite possibly.

In src/extensions/shell/common/shell_content_client.cc, BuildUserAgentFromProduct("Chrome/" PRODUCT_VERSION) is called in ShellContentClient::GetUserAgent. That just circumvents the call to GetShellUserAgent.

Well. There goes the mobile user agent flag. There's other places it's possible for the product to be replaced, but that's the one that sticks out as the culprit.