Lacros: Use CRAS when running on ChromeOS

Add --use-cras flag when it is running on ChromeOS.

BUG=b:151905045

Change-Id: I336a928b512fb1489a82c90a7fd1b776107ef0a8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2374487
Commit-Queue: Yu-Hsuan Hsu <yuhsuan@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801456}

chrome/browser/chromeos/crosapi/browser_manager.cc

@@ -196,6 +196,10 @@ bool BrowserManager::Start() {
                                    "--enable-crashpad",
                                    "--breakpad-dump-location=" + crash_dir};
 
+  // CrAS is the default audio server in Chrome OS.
+  if (base::SysInfo::IsRunningOnChromeOS())
+    argv.push_back("--use-cras");
+
   std::string additional_flags =
       base::CommandLine::ForCurrentProcess()->GetSwitchValueASCII(
           chromeos::switches::kLacrosChromeAdditionalArgs);