Fix initializing volume manager in Files app.

Promise.all was called with a double array [[p1, p2]], instead of [p1, p2],
which caused completing initialization of all volumes before they are added
to the volume list, what caused failing tests in: crrev.com/985533004.

TEST=Fixes test failures in crrev.com/985533004.
BUG=None

Review URL: https://codereview.chromium.org/1020113002

Cr-Commit-Position: refs/heads/master@{#321494}

ui/file_manager/file_manager/background/js/volume_manager.js

@@ -675,7 +675,7 @@ VolumeManager.prototype.initialize_ = function(callback) {
     // volumes in the volumeMetadataList are mounted. crbug.com/135477.
     this.mountQueue_.run(function(inCallback) {
       // Create VolumeInfo for each volume.
-      Promise.all([
+      Promise.all(
           volumeMetadataList.map(function(volumeMetadata) {
             console.debug(
                 'Initializing volume: ' + volumeMetadata.volumeId);
@@ -683,8 +683,8 @@ VolumeManager.prototype.initialize_ = function(callback) {
                 function(volumeInfo) {
                   console.debug('Initialized volume: ' + volumeInfo.volumeId);
                 });
-        }.bind(this))
-      ]).then(function() {
+          }.bind(this)))
+          .then(function() {
             console.debug('Initialized all volumes.');
             // Call the callback of the initialize function.
             callback();