Fix race condition at browser launch re. cosmetic filtering

Related issue:
- https://github.com/uBlockOrigin/uBlock-issues/issues/974

Related feedback:
- https://www.reddit.com/r/uBlockOrigin/comments/fuscia/

The race condition was that a content script could
query the main process to retrieve cosmetic filters
while the cosmetic filters had not been yet fully
loaded into memory. The fix ensure that an already
injected content script will re-query once the
cosmetic filters are fully loaded in memory at
browser launch time.

src/js/background.js

@@ -165,6 +165,10 @@ const µBlock = (( ) => { // jshint ignore:line
         selectedFilterLists: [],
         availableFilterLists: {},
 
+        // https://github.com/uBlockOrigin/uBlock-issues/issues/974
+        //   This can be used to defer filtering decision-making.
+        readyToFilter: false,
+
         pageStores: new Map(),
         pageStoresToken: 0,
 

src/js/messaging.js

@@ -509,6 +509,7 @@ vAPI.messaging.listen({
 const µb = µBlock;
 
 const retrieveContentScriptParameters = function(senderDetails, request) {
+    if ( µb.readyToFilter !== true ) { return; }
     const { url, tabId, frameId } = senderDetails;
     if ( url === undefined || tabId === undefined || frameId === undefined ) {
         return;

src/js/scriptlets/should-inject-contentscript.js

@@ -27,7 +27,7 @@
 // https://github.com/uBlockOrigin/uBlock-issues/issues/403
 //   If the content script was not boostrapped, give it another try.
 
-(function() {
+(( ) => {
     try {
         let status = vAPI.uBO !== true;
         if ( status === false && vAPI.bootstrap ) {

src/js/start.js

@@ -333,6 +333,10 @@ if ( selfieIsValid !== true ) {
 
 // Final initialization steps after all needed assets are in memory.
 
+// https://github.com/uBlockOrigin/uBlock-issues/issues/974
+//   This can be used to defer filtering decision-making.
+µb.readyToFilter = true;
+
 // Start network observers.
 µb.webRequest.start();