[js] Add Enable Mobile feature for Firefox

Still need to expand this to chromium based browsers

Author: AutomatedTester

javascript/node/selenium-webdriver/firefox.js

@@ -222,7 +222,7 @@ async function buildProfile(template, extensions) {
     dir = await io.tmpDir()
     if (template) {
       await io.copyDir(
-        /** @type {string} */ (template),
+        /** @type {string} */(template),
         dir,
         /(parent\.lock|lock|\.parentlock)/
       )
@@ -393,6 +393,24 @@ class Options extends Capabilities {
     }
     throw TypeError('binary must be a string path or Channel object')
   }
+
+  /**
+   * Enables Mobile start up features
+   *
+   * @param {string} androidPackage The package to use
+   * @return {!Options} A self reference
+   */
+  enableMobile(androidPackage = "org.mozilla.firefox", androidActivity = null, deviceSerial = null) {
+    this.firefoxOptions_().androidPackage = androidPackage
+
+    if (androidActivity) {
+      this.firefoxOptions_().androidActivity = androidActivity
+    }
+    if (deviceSerial) {
+      this.firefoxOptions_().deviceSerial = deviceSerial
+    }
+    return this
+  }
 }
 
 /**
@@ -432,11 +450,11 @@ function findGeckoDriver() {
   if (!exe) {
     throw Error(
       'The ' +
-        GECKO_DRIVER_EXE +
-        ' executable could not be found on the current ' +
-        'PATH. Please download the latest version from ' +
-        'https://github.com/mozilla/geckodriver/releases/ ' +
-        'and ensure it can be found on your PATH.'
+      GECKO_DRIVER_EXE +
+      ' executable could not be found on the current ' +
+      'PATH. Please download the latest version from ' +
+      'https://github.com/mozilla/geckodriver/releases/ ' +
+      'and ensure it can be found on your PATH.'
     )
   }
   return exe
@@ -457,8 +475,8 @@ function findInProgramFiles(file) {
     return exists
       ? files[0]
       : io.exists(files[1]).then(function (exists) {
-          return exists ? files[1] : null
-        })
+        return exists ? files[1] : null
+      })
   })
 }
 
@@ -592,7 +610,7 @@ class Driver extends webdriver.WebDriver {
    * implementation.
    * @override
    */
-  setFileDetector() {}
+  setFileDetector() { }
 
   /**
    * Get the context that is currently in effect.

javascript/node/selenium-webdriver/test/firefox_test.js

@@ -95,6 +95,14 @@ suite(
           })
         })
 
+        describe('set mobile options', function () {
+          it('allows setting android activity', function () {
+            let options = new firefox.Options().enableMobile()
+            let firefoxOptions = options.firefoxOptions_()
+            assert.deepStrictEqual({ "androidPackage": "org.mozilla.firefox" }, firefoxOptions)
+          })
+        })
+
         describe('setPreference', function () {
           it('throws if key is not a string', function () {
             let options = new firefox.Options()