Immediately open MTP and other eligible removable devices with media directory on mount.

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

Index: ui/file_manager/file_manager/background/js/volume_manager.js
diff --git a/ui/file_manager/file_manager/background/js/volume_manager.js b/ui/file_manager/file_manager/background/js/volume_manager.js
index 329527a4d6ed61d7671219994a9c5630f96eebc7..660ec4dab53c75834ad6b56e89ffb1f6a5d03680 100644
--- a/ui/file_manager/file_manager/background/js/volume_manager.js
+++ b/ui/file_manager/file_manager/background/js/volume_manager.js
@@ -46,6 +46,8 @@ function VolumeInfo(
   this.displayRoot_ = null;
   this.fakeEntries_ = {};
   this.displayRoot_ = null;
+
+  /** @type {Promise.<DirectoryEntry>} */
   this.displayRootPromise_ = null;
 
   if (volumeType === VolumeManagerCommon.VolumeType.DRIVE) {
@@ -168,7 +170,7 @@ VolumeInfo.prototype = /** @struct */ {
  * @param {function(DirectoryEntry)=} opt_onSuccess Success callback with the
  *     display root directory as an argument.
  * @param {function(*)=} opt_onFailure Failure callback.
- * @return {Promise}
+ * @return {Promise.<DirectoryEntry>}
  */
 VolumeInfo.prototype.resolveDisplayRoot = function(opt_onSuccess,
                                                    opt_onFailure) {
@@ -177,9 +179,11 @@ VolumeInfo.prototype.resolveDisplayRoot = function(opt_onSuccess,
     // remove this if logic. Call opt_onSuccess() always, instead.
     if (this.volumeType !== VolumeManagerCommon.VolumeType.DRIVE) {
       if (this.fileSystem_)
-        this.displayRootPromise_ = Promise.resolve(this.fileSystem_.root);
+        this.displayRootPromise_ = /** @type {Promise.<DirectoryEntry>} */ (
+            Promise.resolve(this.fileSystem_.root));
       else
-        this.displayRootPromise_ = Promise.reject(this.error);
+        this.displayRootPromise_ = /** @type {Promise.<DirectoryEntry>} */ (
+            Promise.reject(this.error));
     } else {
       // For Drive, we need to resolve.
       var displayRootURL = this.fileSystem_.root.toURL() + '/root';
@@ -351,6 +355,9 @@ function VolumeInfoList() {
                                  (volumeManagerUtil.compareVolumeInfo_));
   this.model_.sort(field, 'asc');
 
+  /** @private {!Object.<string, !importer.Resolver.<!VolumeInfo>>} */
+  this.volumeInfoResolvers_ = {};
+
   Object.freeze(this);
 }
 
@@ -387,6 +394,14 @@ VolumeInfoList.prototype.add = function(volumeInfo) {
     this.model_.splice(index, 1, volumeInfo);
   else
     this.model_.push(volumeInfo);
+
+  // Some folks might be expecting this volume to be initialized soon.
+  // In that case there'll be a resolver function (associated with a promise)
+  // waiting for us here.
+  var resolver = this.volumeInfoResolvers_[volumeInfo.volumeId];
+  if (resolver) {
+    resolver.resolve(volumeInfo);
+  }
 };
 
 /**
@@ -397,6 +412,9 @@ VolumeInfoList.prototype.remove = function(volumeId) {
   var index = this.findIndex(volumeId);
   if (index !== -1)
     this.model_.splice(index, 1);
+
+  // Remove any associated resolvers.
+  delete this.volumeInfoResolvers_[volumeId];
 };
 
 /**
@@ -451,6 +469,43 @@ VolumeInfoList.prototype.findByDevicePath = function(devicePath) {
 };
 
 /**
+ * Returns a VolumInfo for the volume ID, or null if not found.
+ *
+ * @param {string} volumeId
+ * @return {VolumeInfo} The volume's information, or null if not found.
+ * @private
+ */
+VolumeInfoList.prototype.findByVolumeId_ = function(volumeId) {
+  var index = this.findIndex(volumeId);
+  return (index !== -1) ?
+      /** @type {VolumeInfo} */ (this.model_.item(index)) :
+      null;
+};
+
+/**
+ * Returns a promise that will be resolved when volume info, identified
+ * by {@code volumeId} is created.
+ *
+ * @param {string} volumeId
+ * @return {!Promise.<!VolumeInfo>} The VolumeInfo. Will not resolve
+ *     if the volume is never mounted.
+ */
+VolumeInfoList.prototype.whenVolumeInfoReady = function(volumeId) {
+  var info = this.findByVolumeId_(volumeId);
+  if (!!info) {
+    return Promise.resolve(info);
+  }
+
+  var resolver = this.volumeInfoResolvers_[volumeId];
+  if (!resolver) {
+    resolver = new importer.Resolver();
+    this.volumeInfoResolvers_[volumeId] = resolver;
+  }
+
+  return resolver.promise;
+};
+
+/**
  * @param {number} index The index of the volume in the list.
  * @return {!VolumeInfo} The VolumeInfo instance.
  */