Bug 1256439 - Run implicit wait callback once before start polling. r=automatedtester

By using a timer the first invocation of the callback is happening after the specified interval.
That means by a default of 100ms we loose that time on every call of implicit wait, and it will
slow-down the test execution drastically. To prevent this lets manually invoke the callback
before starting the timer.

MozReview-Commit-ID: Ayy1GPEaY92

--HG--
extra : rebase_source : 2fac7b5578ec42af5050fad21ca9c3d5cfdca3e1

testing/marionette/element.js

@@ -672,7 +672,7 @@ function implicitlyWaitFor(func, timeout, interval = 100) {
     let startTime = new Date().getTime();
     let endTime = startTime + timeout;
 
-    let observer = function() {
+    let onTimer = function() {
       let res;
       try {
         res = func();
@@ -692,7 +692,11 @@ function implicitlyWaitFor(func, timeout, interval = 100) {
       }
     };
 
-    timer.init(observer, interval, Ci.nsITimer.TYPE_REPEATING_SLACK);
+    // Run a check immediately so we do not cause a delay in execution
+    // due to the set timer interval.
+    onTimer();
+
+    timer.init(onTimer, interval, Ci.nsITimer.TYPE_REPEATING_SLACK);
 
   // cancel timer and return result for yielding
   }).then(res => {