Add longest wait time when fetching from storage in support info

Related issue:
- https://github.com/uBlockOrigin/uBlock-issues/issues/1865

This may be useful to rule out that the issue is caused by the
browser API responding in unduly delayed manner.

src/js/assets.js

@@ -538,6 +538,7 @@ const saveAssetCacheRegistry = (( ) => {
 })();
 
 const assetCacheRead = async function(assetKey, updateReadTime = false) {
+    const t0 = Date.now();
     const internalKey = `cache/${assetKey}`;
 
     const reportBack = function(content) {
@@ -551,6 +552,12 @@ const assetCacheRead = async function(assetKey, updateReadTime = false) {
         getAssetCacheRegistry(),
         cacheStorage.get(internalKey),
     ]);
+
+    µb.supportStats.maxAssetCacheWait = Math.max(
+        Date.now() - t0,
+        parseInt(µb.supportStats.maxAssetCacheWait, 10)
+    ) + ' ms';
+
     if (
         bin instanceof Object === false ||
         bin.hasOwnProperty(internalKey) === false

src/js/background.js

@@ -215,6 +215,7 @@ const µBlock = {  // jshint ignore:line
 
     supportStats: {
         allReadyAfter: '',
+        maxAssetCacheWait: '0 ms',
     },
 
     pageStores: new Map(),