Replace the objectFromEntries helper function with an objectFromMap one instead

Given that it's only used with `Map`s, and that it's currently implemented in such a way that we (indirectly) must iterate through the data *twice*, some simplification cannot hurt here.
Note that the only reason that we're not using `Object.fromEntries(...)` directly, at each call-site, is that that one won't guarantee that a `null` prototype is being used.

src/display/annotation_storage.js

@@ -14,7 +14,7 @@
  */
 
 import { deprecated } from "./display_utils.js";
-import { objectFromEntries } from "../shared/util.js";
+import { objectFromMap } from "../shared/util.js";
 
 /**
  * Key/value storage for annotation data in forms.
@@ -91,7 +91,7 @@ class AnnotationStorage {
   }
 
   getAll() {
-    return this._storage.size > 0 ? objectFromEntries(this._storage) : null;
+    return this._storage.size > 0 ? objectFromMap(this._storage) : null;
   }
 
   get size() {

src/display/metadata.js

@@ -13,7 +13,7 @@
  * limitations under the License.
  */
 
-import { objectFromEntries } from "../shared/util.js";
+import { objectFromMap } from "../shared/util.js";
 
 class Metadata {
   constructor({ parsedData, rawData }) {
@@ -30,7 +30,7 @@ class Metadata {
   }
 
   getAll() {
-    return objectFromEntries(this._metadataMap);
+    return objectFromMap(this._metadataMap);
   }
 
   has(name) {

src/display/optional_content_config.js

@@ -12,7 +12,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-import { objectFromEntries, warn } from "../shared/util.js";
+import { objectFromMap, warn } from "../shared/util.js";
 
 class OptionalContentGroup {
   constructor(name, intent) {
@@ -142,10 +142,7 @@ class OptionalContentConfig {
   }
 
   getGroups() {
-    if (!this._groups.size) {
-      return null;
-    }
-    return objectFromEntries(this._groups);
+    return this._groups.size > 0 ? objectFromMap(this._groups) : null;
   }
 
   getGroup(id) {

src/shared/util.js

@@ -597,9 +597,14 @@ function objectSize(obj) {
   return Object.keys(obj).length;
 }
 
-// Ensures that the returned Object has a `null` prototype.
-function objectFromEntries(iterable) {
-  return Object.assign(Object.create(null), Object.fromEntries(iterable));
+// Ensure that the returned Object has a `null` prototype; hence why
+// `Object.fromEntries(...)` is not used.
+function objectFromMap(map) {
+  const obj = Object.create(null);
+  for (const [key, value] of map) {
+    obj[key] = value;
+  }
+  return obj;
 }
 
 // Checks the endianness of the platform.
@@ -1006,7 +1011,7 @@ export {
   isSameOrigin,
   isString,
   MissingPDFException,
-  objectFromEntries,
+  objectFromMap,
   objectSize,
   OPS,
   PageActionEventType,