Add benchmark method to cosmetic filtering engine

To measure retrieval of site-specific selectors. From
uBO's own dev console:

    µBlock.cosmeticFilteringEngine.benchmark();

src/js/cosmetic-filtering.js

@@ -1410,6 +1410,45 @@ FilterContainer.prototype.getFilterCount = function() {
 
 /******************************************************************************/
 
+FilterContainer.prototype.benchmark = function() {
+    µb.loadBenchmarkDataset().then(requests => {
+        if ( Array.isArray(requests) === false || requests.length === 0 ) {
+            console.info('No requests found to benchmark');
+            return;
+        }
+        console.info(`Benchmarking cosmeticFilteringEngine.matchString()...`);
+        const details = {
+            tabId: undefined,
+            frameId: undefined,
+            hostname: '',
+            domain: '',
+            entity: '',
+        };
+        const options = {
+            noCosmeticFiltering: false,
+            noGenericCosmeticFiltering: false,
+        };
+        let count = 0;
+        const t0 = self.performance.now();
+        for ( let i = 0; i < requests.length; i++ ) {
+            const request = requests[i];
+            if ( request.cpt !== 'document' ) { continue; }
+            count += 1;
+            details.hostname = µb.URI.hostnameFromURI(request.url);
+            details.domain = µb.URI.domainFromHostname(details.hostname);
+            details.entity = µb.URI.entityFromDomain(details.domain);
+            void this.retrieveSpecificSelectors(details, options);
+        }
+        const t1 = self.performance.now();
+        const dur = t1 - t0;
+        console.info(`Evaluated ${count} requests in ${dur.toFixed(0)} ms`);
+        console.info(`\tAverage: ${(dur / count).toFixed(3)} ms per request`);
+    });
+    return 'ok';
+};
+
+/******************************************************************************/
+
 return new FilterContainer();
 
 /******************************************************************************/