Convert the pendingFindMatches member, in `web/pdf_find_controller.…

…js`, from an object to a set

We only want to track page numbers instead of actual data, so using a
set conveys that intention more clearly and is slightly more efficient.

web/pdf_find_controller.js

@@ -249,7 +249,7 @@ class PDFFindController {
     this._pageDiffs = [];
     this._matchesCountTotal = 0;
     this._pagesToSearch = null;
-    this._pendingFindMatches = Object.create(null);
+    this._pendingFindMatches = new Set();
     this._resumePageIdx = null;
     this._dirtyMatch = false;
     clearTimeout(this._findTimeout);
@@ -600,12 +600,12 @@ class PDFFindController {
 
       for (let i = 0; i < numPages; i++) {
         // Start finding the matches as soon as the text is extracted.
-        if (this._pendingFindMatches[i] === true) {
+        if (this._pendingFindMatches.has(i)) {
           continue;
         }
-        this._pendingFindMatches[i] = true;
+        this._pendingFindMatches.add(i);
         this._extractTextPromises[i].then(pageIdx => {
-          delete this._pendingFindMatches[pageIdx];
+          this._pendingFindMatches.delete(pageIdx);
           this._calculateMatch(pageIdx);
         });
       }