Do not flush blocked-elements cache at webNavigation time

While testing for the new `elemhide` option using ABP's
test page[1], I found out that the placeholder of the
blocked image on that page was not properly collapsed
by uBO.

The reason was because the page is very simple and
flushing the blocked-elements cache at
webNavigation.onCommitted time was causing the loss
of information collected between
webRequest.onBeforeRequest and
webNavigation.onCommitted, preventing uBO from
properly collapsing the placeholders.

The blocked-elements cache is now flushed
ONLY at webRequest.onBeforeRequest time.

[1] https://testpages.adblockplus.org/en/exceptions/elemhide

src/js/pagestore.js

@@ -41,10 +41,6 @@ const µb = µBlock;
 
 /******************************************************************************/
 
-// To mitigate memory churning
-const netFilteringCacheJunkyard = [];
-const netFilteringCacheJunkyardMax = 10;
-
 const NetFilteringResultCache = class {
     constructor() {
         this.init();
@@ -58,14 +54,6 @@ const NetFilteringResultCache = class {
         return this;
     }
 
-    dispose() {
-        this.empty();
-        if ( netFilteringCacheJunkyard.length < netFilteringCacheJunkyardMax ) {
-            netFilteringCacheJunkyard.push(this);
-        }
-        return null;
-    }
-
     rememberResult(fctxt, result) {
         if ( fctxt.tabId <= 0 ) { return; }
         if ( this.results.size === 0 ) {
@@ -155,10 +143,7 @@ const NetFilteringResultCache = class {
     }
 
     static factory() {
-        const entry = netFilteringCacheJunkyard.pop();
-        return entry !== undefined
-            ? entry.init()
-            : new NetFilteringResultCache();
+        return new NetFilteringResultCache();
     }
 };
 
@@ -218,6 +203,7 @@ const PageStore = class {
         this.journalTimer = null;
         this.journalLastCommitted = this.journalLastUncommitted = undefined;
         this.journalLastUncommittedURL = undefined;
+        this.netFilteringCache = NetFilteringResultCache.factory();
         this.init(tabId, context);
     }
 
@@ -261,7 +247,6 @@ const PageStore = class {
         this.popupBlockedCount = 0;
         this.largeMediaCount = 0;
         this.largeMediaTimer = null;
-        this.netFilteringCache = NetFilteringResultCache.factory();
         this.internalRedirectionCount = 0;
         this.extraData.clear();
 
@@ -327,7 +312,6 @@ const PageStore = class {
             this.largeMediaTimer = null;
         }
         this.disposeFrameStores();
-        this.netFilteringCache = this.netFilteringCache.dispose();
         this.init(this.tabId, context);
         return this;
     }
@@ -337,13 +321,13 @@ const PageStore = class {
         this.title = '';
         this.rawURL = '';
         this.hostnameToCountMap = null;
+        this.netFilteringCache.empty();
         this.allowLargeMediaElementsUntil = 0;
         if ( this.largeMediaTimer !== null ) {
             clearTimeout(this.largeMediaTimer);
             this.largeMediaTimer = null;
         }
         this.disposeFrameStores();
-        this.netFilteringCache = this.netFilteringCache.dispose();
         if ( this.journalTimer !== null ) {
             clearTimeout(this.journalTimer);
             this.journalTimer = null;

src/js/tab.js

@@ -887,6 +887,7 @@ vAPI.tabs = new vAPI.Tabs();
     // https://github.com/chrisaljoudi/uBlock/issues/516
     //   If context is 'beforeRequest', do not rebind, wait for confirmation.
     if ( context === 'beforeRequest' ) {
+        pageStore.netFilteringCache.empty();
         return pageStore;
     }