Remember the last volume level in Video Player.

ui/file_manager/video_player/js/media_controls.js

Index: ui/file_manager/video_player/js/media_controls.js
diff --git a/ui/file_manager/video_player/js/media_controls.js b/ui/file_manager/video_player/js/media_controls.js
index a2b44eb160d9e17ab3ce523e4c8e10ae94bce0ca..f5d456cf40e84934f31e5ec6d5c74885db623f32 100644
--- a/ui/file_manager/video_player/js/media_controls.js
+++ b/ui/file_manager/video_player/js/media_controls.js
@@ -489,6 +489,10 @@ MediaControls.prototype.updateTimeLabel_ = function(current, opt_duration) {
  * Volume controls
  */
 
+MediaControls.STORAGE_PREFIX = 'videoplayer-';
+
+MediaControls.KEY_VOLUME = MediaControls.STORAGE_PREFIX + 'volume';
+
 /**
  * @param {HTMLElement=} opt_parent Parent element for the controls.
  */
@@ -512,10 +516,24 @@ MediaControls.prototype.initVolumeControls = function(opt_parent) {
   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
@@ -559,6 +577,7 @@ MediaControls.prototype.onVolumeChange_ = function(value) {
   this.soundButton_.setAttribute('aria-label',
       value === 0 ? str('MEDIA_PLAYER_UNMUTE_BUTTON_LABEL')
                   : str('MEDIA_PLAYER_MUTE_BUTTON_LABEL'));
+  this.saveVolumeControlState();
 };
 
 /**