Remove watchers spam from log.

Not supporting file watchers is not an error. It's expected from eg. file
systems exposed via File System Provider API, especially those which are
read only.

TEST=Tested manually that watchers work and there is no spam in the log.
BUG=461735

Review URL: https://codereview.chromium.org/1150473002

Cr-Commit-Position: refs/heads/master@{#330697}

ui/file_manager/file_manager/foreground/js/directory_model.js

@@ -945,9 +945,8 @@ DirectoryModel.prototype.changeDirectoryEntry = function(
     this.currentDirContents_.cancelScan();
 
   this.directoryChangeQueue_.run(function(sequence, queueTaskCallback) {
-    this.fileWatcher_.changeWatchedDirectory(
-        dirEntry,
-        function() {
+    this.fileWatcher_.changeWatchedDirectory(dirEntry)
+        .then(function() {
           if (this.changeDirectorySequence_ !== sequence) {
             queueTaskCallback();
             return;

ui/file_manager/file_manager/foreground/js/file_watcher.js

@@ -29,7 +29,7 @@ FileWatcher.prototype.dispose = function() {
   chrome.fileManagerPrivate.onDirectoryChanged.removeListener(
       this.onDirectoryChangedBound_);
   if (this.watchedDirectoryEntry_)
-    this.resetWatchedEntry_(function() {}, function() {});
+    this.resetWatchedEntry_();
 };
 
 /**
@@ -71,87 +71,80 @@ FileWatcher.prototype.onDirectoryChanged_ = function(event) {
  *
  * @param {!DirectoryEntry|!FakeEntry} entry Directory entry to be tracked, or
  *     the fake entry.
- * @param {function()} callback Completion callback.
+ * @return {!Promise}
  */
-FileWatcher.prototype.changeWatchedDirectory = function(entry, callback) {
-  if (!util.isFakeEntry(entry)) {
-    this.changeWatchedEntry_(
-        /** @type {!DirectoryEntry} */ (entry),
-        callback,
-        function() {
-          console.error(
-              'Unable to change the watched directory to: ' + entry.toURL());
-          callback();
-        });
-  } else {
-    this.resetWatchedEntry_(
-        callback,
-        function() {
-          console.error('Unable to reset the watched directory.');
-          callback();
-        });
-  }
+FileWatcher.prototype.changeWatchedDirectory = function(entry) {
+  if (!util.isFakeEntry(entry))
+    return this.changeWatchedEntry_(/** @type {!DirectoryEntry} */ (entry));
+  else
+    return this.resetWatchedEntry_();
 };
 
 /**
- * Resets the watched entry to the passed directory.
- *
- * @param {function()} onSuccess Success callback.
- * @param {function()} onError Error callback.
+ * Resets the watched entry. It's a best effort method.
+ * @return {!Promise}
  * @private
  */
-FileWatcher.prototype.resetWatchedEntry_ = function(onSuccess, onError) {
+FileWatcher.prototype.resetWatchedEntry_ = function() {
   // Run the tasks in the queue to avoid races.
-  this.queue_.run(function(callback) {
-    // Release the watched directory.
-    if (this.watchedDirectoryEntry_) {
-      chrome.fileManagerPrivate.removeFileWatch(
-          this.watchedDirectoryEntry_.toURL(),
-          function(result) {
-            this.watchedDirectoryEntry_ = null;
-            if (result)
-              onSuccess();
-            else
-              onError();
-            callback();
-          }.bind(this));
-    } else {
-      onSuccess();
-      callback();
-    }
+  return new Promise(function(fulfill, reject) {
+    this.queue_.run(function(callback) {
+      // Release the watched directory.
+      if (this.watchedDirectoryEntry_) {
+        chrome.fileManagerPrivate.removeFileWatch(
+            this.watchedDirectoryEntry_.toURL(),
+            function(result) {
+              if (chrome.runtime.lastError) {
+                console.error('Failed to remove the watcher because of: ' +
+                    chrome.runtime.lastError.message);
+              }
+              // Even on error reset the watcher locally, so at least the
+              // notifications are discarded.
+              this.watchedDirectoryEntry_ = null;
+              fulfill();
+              callback();
+            }.bind(this));
+      } else {
+        fulfill();
+        callback();
+      }
+    }.bind(this));
   }.bind(this));
 };
 
 /**
- * Sets the watched entry to the passed directory.
- *
+ * Sets the watched entry to the passed directory. It's a best effort method.
  * @param {!DirectoryEntry} entry Directory to be watched.
- * @param {function()} onSuccess Success callback.
- * @param {function()} onError Error callback.
+ * @return {!Promise}
  * @private
  */
-FileWatcher.prototype.changeWatchedEntry_ = function(
-    entry, onSuccess, onError) {
-  var setEntryClosure = function() {
-    // Run the tasks in the queue to avoid races.
-    this.queue_.run(function(callback) {
-      chrome.fileManagerPrivate.addFileWatch(
-          entry.toURL(),
-          function(result) {
-            if (!result) {
-              this.watchedDirectoryEntry_ = null;
-              onError();
-            } else {
-              this.watchedDirectoryEntry_ = entry;
-              onSuccess();
-            }
-            callback();
-          }.bind(this));
-    }.bind(this));
-  }.bind(this);
+FileWatcher.prototype.changeWatchedEntry_ = function(entry) {
+  return new Promise(function(fulfill, reject) {
+    var setEntryClosure = function() {
+      // Run the tasks in the queue to avoid races.
+      this.queue_.run(function(callback) {
+        chrome.fileManagerPrivate.addFileWatch(
+            entry.toURL(),
+            function(result) {
+              if (chrome.runtime.lastError) {
+                // Most probably setting the watcher is not supported on the
+                // file system type.
+                console.info('File watchers not supported for: ' +
+                    entry.toURL());
+                this.watchedDirectoryEntry_ = null;
+                fulfill();
+              } else {
+                this.watchedDirectoryEntry_ = assert(entry);
+                fulfill();
+              }
+              callback();
+            }.bind(this));
+      }.bind(this));
+    }.bind(this);
 
-  // Reset the watched directory first, then set the new watched directory.
-  this.resetWatchedEntry_(setEntryClosure, onError);
+    // Reset the watched directory first, then set the new watched directory.
+    return this.resetWatchedEntry_().then(setEntryClosure);
+  }.bind(this));
 };
 
 /**