[fsp] Polish icons in Files app.

This patch makes the icons smaller plus fixes the mask, so they are always
white (except transparent pixels) when selected.

TBR=fukino@chromium.org
TEST=Tested manually with 3 different icons.
BUG=412602

Review URL: https://codereview.chromium.org/693073002

Cr-Commit-Position: refs/heads/master@{#302224}

ui/file_manager/file_manager/foreground/css/file_types.css

@@ -511,5 +511,5 @@ tree:focus .tree-item[selected] > .tree-row >
 
 list:focus li[selected] [volume-type-icon='provided'],
 tree:focus .tree-item[selected] > .tree-row > [volume-type-icon='provided']{
-  -webkit-filter: invert(100%) grayscale(100%) brightness(200%);
+  -webkit-filter: contrast(0) brightness(200%);
 }

ui/file_manager/file_manager/foreground/js/ui/directory_tree.js

@@ -626,12 +626,12 @@ VolumeItem.prototype.isRemovable_ = function() {
  */
 VolumeItem.prototype.setupIcon_ = function(icon, volumeInfo) {
   icon.classList.add('volume-icon');
-  if (volumeInfo.volumeType === 'provided') {
+  if (volumeInfo.volumeType === VolumeManagerCommon.VolumeType.PROVIDED) {
     var backgroundImage = '-webkit-image-set(' +
         'url(chrome://extension-icon/' + volumeInfo.extensionId +
-            '/24/1) 1x, ' +
+            '/16/1) 1x, ' +
         'url(chrome://extension-icon/' + volumeInfo.extensionId +
-            '/48/1) 2x);';
+            '/32/1) 2x);';
     // The icon div is not yet added to DOM, therefore it is impossible to
     // use style.backgroundImage.
     icon.setAttribute(