kill chromedriver before starting new chromedriver

lib/devices/android/chrome.js

@@ -36,6 +36,7 @@ ChromeAndroid.prototype.start = function(cb, onDie) {
   async.waterfall([
     this.ensureChromedriverExists.bind(this),
     this.unlock.bind(this),
+    this.killOldChromedrivers.bind(this),
     this.startChromedriver.bind(this),
     this.createSession.bind(this)
   ], cb);
@@ -58,6 +59,19 @@ ChromeAndroid.prototype.ensureChromedriverExists = function(cb) {
   }.bind(this));
 };
 
+ChromeAndroid.prototype.killOldChromedrivers = function(cb) {
+  var cmd = isWindows ? "TASKKILL /IM chromedriver.exe" : "killall chromedriver";
+  logger.info("Killing any old chromedrivers");
+  exec(cmd, function(err) {
+    if (err) {
+      logger.info("No old chromedrivers seemed to exist");
+    } else {
+      logger.info("Successfully cleaned up old chromedrivers");
+    }
+    cb();
+  });
+};
+
 ChromeAndroid.prototype.startChromedriver = function(cb) {
   this.onChromedriverStart = cb;
   logger.info("Spawning chromedriver with: " + this.chromedriver);