Remove CSS variables feature-testing from PDFSidebarResizer

CSS variables are now supported in all reasonably modern browsers, according to:
 - https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_custom_properties#Browser_compatibility
 - https://caniuse.com/css-variables

web/pdf_sidebar_resizer.js

@@ -34,7 +34,7 @@ class PDFSidebarResizer {
    * @param {IL10n} l10n - Localization service.
    */
   constructor(options, eventBus, l10n = NullL10n) {
-    this.enabled = false;
+    this.enabled = true;
     this.isRTL = false;
     this.sidebarOpen = false;
     this.doc = document.documentElement;
@@ -45,24 +45,8 @@ class PDFSidebarResizer {
     this.outerContainer = options.outerContainer;
     this.resizer = options.resizer;
     this.eventBus = eventBus;
-    this.l10n = l10n;
-
-    if (
-      (typeof PDFJSDev === "undefined" || !PDFJSDev.test("MOZCENTRAL")) &&
-      (typeof CSS === "undefined" ||
-        typeof CSS.supports !== "function" ||
-        !CSS.supports(SIDEBAR_WIDTH_VAR, `calc(-1 * ${SIDEBAR_MIN_WIDTH}px)`))
-    ) {
-      console.warn(
-        "PDFSidebarResizer: " +
-          "The browser does not support resizing of the sidebar."
-      );
-      return;
-    }
-    this.enabled = true;
-    this.resizer.classList.remove("hidden"); // Show the resizer DOM element.
 
-    this.l10n.getDirection().then(dir => {
+    l10n.getDirection().then(dir => {
       this.isRTL = dir === "rtl";
     });
     this._addEventListeners();

web/viewer.html

@@ -101,7 +101,7 @@
           <div id="layersView" class="hidden">
           </div>
         </div>
-        <div id="sidebarResizer" class="hidden"></div>
+        <div id="sidebarResizer"></div>
       </div>  <!-- sidebarContainer -->
 
       <div id="mainContainer">