Replace instanceof Object with typeof checks

Using `instanceof Object` is generally problematic, since it's not guaranteed to always do the right thing for all Objects.
(I stumbled upon this while working on another patch, when I noticed that the `outlineView` was broken with workers disabled.)

.eslintrc

@@ -166,6 +166,10 @@
     "no-nested-ternary": "error",
     "no-new-object": "error",
     "no-restricted-syntax": ["error",
+      {
+        "selector": "BinaryExpression[operator='instanceof'][right.name='Object']",
+        "message": "Use `typeof` rather than `instanceof Object`.",
+      },
       {
         "selector": "CallExpression[callee.name='assert'][arguments.length!=2]",
         "message": "`assert()` must always be invoked with two arguments.",

src/core/xfa/xfa_object.js

@@ -586,7 +586,7 @@ class XFAObject {
     if (Array.isArray(obj)) {
       return obj.map(x => XFAObject[_cloneAttribute](x));
     }
-    if (obj instanceof Object) {
+    if (typeof obj === "object" && obj !== null) {
       return Object.assign({}, obj);
     }
     return obj;

web/pdf_link_service.js

@@ -114,7 +114,7 @@ class PDFLinkService {
     const destRef = explicitDest[0];
     let pageNumber;
 
-    if (destRef instanceof Object) {
+    if (typeof destRef === "object" && destRef !== null) {
       pageNumber = this._cachedPageNumber(destRef);
 
       if (pageNumber === null) {

web/pdf_outline_viewer.js

@@ -304,7 +304,7 @@ class PDFOutlineViewer extends BaseTreeViewer {
         if (Array.isArray(explicitDest)) {
           const [destRef] = explicitDest;
 
-          if (destRef instanceof Object) {
+          if (typeof destRef === "object" && destRef !== null) {
             pageNumber = this.linkService._cachedPageNumber(destRef);
 
             if (!pageNumber) {