Code review related to performance in main content script

- Avoid concatenating with empty array: though the concatenated
  array is empty, this still forces the creation of a whole new
  array as per semantic of Array.prototype.concat().
  <https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/concat>

- Do not convert arrays to strings when sending data to
  main process in surveyPhase1(): I no longer see any benefit
  doing so in profiling data (if I recall properly this was
  benefiting Firefox, but I can't remember for sure anymore why
  I chose to do so back then).

src/js/contentscript.js

@@ -1147,8 +1147,10 @@ vAPI.domSurveyor = (function() {
             pending.nodes = added;
         } else {
             nodes = pending.nodes.splice(0, 1000);
+            if ( added.length !== 0 ) {
                 pending.nodes = pending.nodes.concat(added);
             }
+        }
         return nodes;
     };
 
@@ -1212,8 +1214,8 @@ vAPI.domSurveyor = (function() {
                 {
                     what: 'retrieveGenericCosmeticSelectors',
                     hostname: hostname,
-                    ids: ids.join('\n'),
-                    classes: classes.join('\n'),
+                    ids: ids,
+                    classes: classes,
                     exceptions: domFilterer.exceptions,
                     cost: surveyCost
                 },

src/js/cosmetic-filtering.js

@@ -1054,14 +1054,8 @@ FilterContainer.prototype.retrieveGenericSelectors = function(request) {
     for ( let type in this.lowlyGeneric ) {
         let entry = this.lowlyGeneric[type];
         let selectors = request[entry.canonical];
-        if ( typeof selectors !== 'string' ) { continue; }
-        let strEnd = selectors.length;
-        let sliceBeg = 0;
-        do {
-            let sliceEnd = selectors.indexOf('\n', sliceBeg);
-            if ( sliceEnd === -1 ) { sliceEnd = strEnd; }
-            let selector = selectors.slice(sliceBeg, sliceEnd);
-            sliceBeg = sliceEnd + 1;
+        if ( Array.isArray(selectors) === false ) { continue; }
+        for ( let selector of selectors ) {
             if ( entry.simple.has(selector) === false ) { continue; }
             let bucket = entry.complex.get(selector);
             if ( bucket !== undefined ) {
@@ -1080,7 +1074,7 @@ FilterContainer.prototype.retrieveGenericSelectors = function(request) {
                     simpleSelectors.add(selector);
                 }
             }
-        } while ( sliceBeg < strEnd );
+        }
     }
 
     // Apply exceptions: it is the responsibility of the caller to provide