WebUI: revise hash function

In benchmark, using `Math.imul()` is about ~20% faster than floating
point multiplication.

PR #15475.

src/webui/www/private/scripts/client.js

@@ -88,11 +88,12 @@ const loadSelectedTracker = function() {
 loadSelectedTracker();
 
 function genHash(string) {
+    // origins:
+    // https://stackoverflow.com/a/8831937
+    // https://gist.github.com/hyamamoto/fd435505d29ebfa3d9716fd2be8d42f0
     let hash = 0;
-    for (let i = 0; i < string.length; ++i) {
-        const c = string.charCodeAt(i);
-        hash = (c + hash * 31) | 0;
-    }
+    for (let i = 0; i < string.length; ++i)
+        hash = ((Math.imul(hash, 31) + string.charCodeAt(i)) | 0);
     return hash;
 }