Fixed duplicated chrome startup (#266)

Fixed startup for web execution. Currently chrome browser is started
twice: fristly by: ensureAppStarts and then by startChromeSession. It
can be skipped and done similarly to solution implemented in
AndroidDriver.

lib/driver.js

@@ -334,8 +334,9 @@ class AndroidUiautomator2Driver extends BaseDriver {
       logger.debug(`'skipUnlock' capability set, so skipping device unlock`);
     }
 
-    // rescue UiAutomator2 if it fails to start our AUT
-    if (this.opts.autoLaunch) {
+    if (this.isChromeSession) { // start a chromedriver session
+      await this.startChromeSession(this);
+    } else if (this.opts.autoLaunch) { // rescue UiAutomator2 if it fails to start our AUT
       await this.ensureAppStarts();
     }
 
@@ -354,10 +355,6 @@ class AndroidUiautomator2Driver extends BaseDriver {
       await retryInterval(timeout / 500, 500, this.setContext.bind(this), viewName);
     }
 
-    if (this.isChromeSession) {
-      await this.startChromeSession(this);
-    }
-
     // now that everything has started successfully, turn on proxying so all
     // subsequent session requests go straight to/from uiautomator2
     this.jwpProxyActive = true;