Remember the last volume level in Video Player.

ui/file_manager/video_player/js/media_controls.js

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * @fileoverview MediaControls class implements media playback controls
  * that exist outside of the audio/video HTML element.
  */
 
 /**
@@ skipping 471 matching lines @@
     // not known to be bounded yet. In such cases, we should hide duration.
     this.currentTime_.textContent = MediaControls.formatTime_(current);
     this.currentTimeSpacer_.textContent = MediaControls.formatTime_(current);
   }
 };
 
 /*
  * Volume controls
  */
 
+MediaControls.STORAGE_PREFIX = 'videoplayer-';
+
+MediaControls.KEY_VOLUME = MediaControls.STORAGE_PREFIX + 'volume';
+
 /**
  * @param {HTMLElement=} opt_parent Parent element for the controls.
  */
 MediaControls.prototype.initVolumeControls = function(opt_parent) {
   var volumeControls = this.createControl('volume-controls', opt_parent);
 
   this.soundButton_ = this.createButton('sound media-control',
       this.onSoundButtonClick_.bind(this), volumeControls);
   this.soundButton_.setAttribute('level', 3);  // max level.
   this.soundButton_.setAttribute('aria-label',
       str('MEDIA_PLAYER_MUTE_BUTTON_LABEL'));
 
   this.volume_ = /** @type {!PaperSliderElement} */ (
       document.createElement('paper-slider'));
   this.volume_.classList.add('volume', 'media-control');
   this.volume_.setAttribute('aria-label',
       str('MEDIA_PLAYER_VOLUME_SLIDER_LABEL'));
   this.volume_.addEventListener('change', function(event) {
     this.onVolumeChange_(this.volume_.ratio);
   }.bind(this));
   this.volume_.addEventListener('immediate-value-change', function(event) {
     this.onVolumeDrag_();
   }.bind(this));
-  this.volume_.value = this.volume_.max;
+  this.loadVolumeControlState();
   volumeControls.appendChild(this.volume_);
 };
 
+MediaControls.prototype.loadVolumeControlState = function() {
+  chrome.storage.local.get([MediaControls.KEY_VOLUME], function(retrieved) {
+    var volume = (MediaControls.KEY_VOLUME in retrieved)
+        ? retrieved[MediaControls.KEY_VOLUME] : 1;

[oka, 2016/11/29 05:58:41]

retrieved[MediaControls.KEY_VOLUME] || 1;
is common construct IMO.

[yamaguchi, 2016/11/29 06:51:32]

The stored volume can be 0 when muted or the slider is moved to the minimum.

[oka, 2016/11/29 07:13:57]

Ah. Sorry.

+    this.volume_.value = this.volume_.max * volume;
+  }.bind(this));
+};
+
+MediaControls.prototype.saveVolumeControlState = function() {
+  var valuesToStore = {};
+  valuesToStore[MediaControls.KEY_VOLUME] = this.media_.volume;

[oka, 2016/11/29 05:58:41]

Optional: media_.volume actually means the ratio of volume/max. How about
renaming volume to volumeRatio while we are here?

[yamaguchi, 2016/11/29 06:51:32]

Added "normalized" to those symbol names.
I also think it's safer to declare it's a [0,1] value because media_ has a [0,1]
volume whereas the PaperSlider (this.volume_) has [0,100] volume.

+  chrome.storage.local.set(valuesToStore, function() {});

[oka, 2016/11/29 05:58:41]

Remove the callback, which is optional.

[yamaguchi, 2016/11/29 06:51:32]

Done.

+};
+
 /**
  * Click handler for the sound level button.
  * @private
  */
 MediaControls.prototype.onSoundButtonClick_ = function() {
   if (this.media_.volume == 0) {
     this.volume_.value = (this.savedVolume_ || 1) * this.volume_.max;
     this.soundButton_.setAttribute('aria-label',
         str('MEDIA_PLAYER_MUTE_BUTTON_LABEL'));
   } else {
@@ skipping 23 matching lines @@
  */
 MediaControls.prototype.onVolumeChange_ = function(value) {
   if (!this.media_)
     return;  // Media is detached.
 
   this.media_.volume = value;
   this.soundButton_.setAttribute('level', MediaControls.getVolumeLevel_(value));
   this.soundButton_.setAttribute('aria-label',
       value === 0 ? str('MEDIA_PLAYER_UNMUTE_BUTTON_LABEL')
                   : str('MEDIA_PLAYER_MUTE_BUTTON_LABEL'));
+  this.saveVolumeControlState();
 };
 
 /**
  * @private
  */
 MediaControls.prototype.onVolumeDrag_ = function() {
   if (this.media_.volume !== 0) {
     this.savedVolume_ = this.media_.volume;
   }
 };
@@ skipping 521 matching lines @@
     this.fullscreenButton_.setAttribute('aria-label',
         fullscreen ? str('VIDEO_PLAYER_EXIT_FULL_SCREEN_BUTTON_LABEL')
                    : str('VIDEO_PLAYER_FULL_SCREEN_BUTTON_LABEL'));;
     // If the fullscreen button has focus on entering fullscreen mode, reset the
     // focus to make the spacebar toggle play/pause state. This is the
     // consistent behavior with Youtube Web UI.
     if (fullscreen)
       this.fullscreenButton_.blur();
   }
 };