Use devtools to set user agent in chromedriver

chrome/test/chromedriver/capabilities.h

Index: chrome/test/chromedriver/capabilities.h
diff --git a/chrome/test/chromedriver/capabilities.h b/chrome/test/chromedriver/capabilities.h
index 22ede2b2f4f157a133d9e0f2c6418376da4700f8..450a9e5a1756a24f239693ff035c68f105888140 100644
--- a/chrome/test/chromedriver/capabilities.h
+++ b/chrome/test/chromedriver/capabilities.h
@@ -154,6 +154,9 @@ struct Capabilities {
 
   Switches switches;
 
+  // If set, use this as user agent for device emulation.
+  std::string user_agent;
+
   std::set<WebViewInfo::Type> window_types;
 };