feat(Launcher): allow the default arguments to be overridden (#1623)

This patch:
- adds `puppeteer.defaultArgs()` method to get default arguments that are used to launch chrome
- adds `ignoreDefaultArgs` option to `puppeteer.launch` to avoid using default puppeteer arguments

Fixes #872

lib/Launcher.js

@@ -61,10 +61,13 @@ class Launcher {
   static async launch(options) {
     options = Object.assign({}, options || {});
     let temporaryUserDataDir = null;
-    const chromeArguments = [].concat(DEFAULT_ARGS);
+    const chromeArguments = [];
+    if (!options.ignoreDefaultArgs)
+      chromeArguments.push(...DEFAULT_ARGS);
+
     if (options.appMode)
       options.headless = false;
-    else
+    else if (!options.ignoreDefaultArgs)
       chromeArguments.push(...AUTOMATION_ARGS);
 
     if (!options.args || !options.args.some(arg => arg.startsWith('--user-data-dir'))) {
@@ -172,6 +175,13 @@ class Launcher {
     }
   }
 
+  /**
+   * @return {!Array<string>}
+   */
+  static defaultArgs() {
+    return DEFAULT_ARGS.concat(AUTOMATION_ARGS);
+  }
+
   /**
    * @return {string}
    */