Add ability to defer set-constant execution

A new optional parameter has been added to `set-constant`
scriptlet: `runAt`, default to `0`.

 ..##+js(set, document.body.oncontextmenu, null, 2)

When the `runAt` parameter is present, uBO will take it into
account to possibly defer execution of `set-constant`:

- `runAt` not present: execute immediately
- `runAt` = 1: execute immediately
- `runAt` = 2: execute when document state is "interactive"
- `runAt` = 3: execute when document state is `"complete"

assets/resources/scriptlets.js

@@ -982,11 +982,17 @@ builtinScriptlets.push({
     fn: setConstant,
 });
 function setConstant(
-    chain = '',
-    cValue = ''
+    arg1 = '',
+    arg2 = '',
+    arg3 = 0
 ) {
+    const details = typeof arg1 !== 'object'
+        ? { prop: arg1, value: arg2, runAt: parseInt(arg3, 10) || 0 }
+        : arg1;
+    const { prop: chain = '', value: cValue = '' } = details;
     if ( typeof chain !== 'string' ) { return; }
     if ( chain === '' ) { return; }
+    function setConstant(chain, cValue) {
         const trappedProp = (( ) => {
             const pos = chain.lastIndexOf('.');
             if ( pos === -1 ) { return chain; }
@@ -1060,7 +1066,7 @@ function setConstant(
         // https://github.com/uBlockOrigin/uBlock-issues/issues/156
         //   Support multiple trappers for the same property.
         const trapProp = function(owner, prop, configurable, handler) {
-        if ( handler.init(owner[prop]) === false ) { return; }
+            if ( handler.init(configurable ? owner[prop] : cValue) === false ) { return; }
             const odesc = Object.getOwnPropertyDescriptor(owner, prop);
             let prevGetter, prevSetter;
             if ( odesc instanceof Object ) {
@@ -1139,6 +1145,23 @@ function setConstant(
         };
         trapChain(window, chain);
     }
+    const runAt = details.runAt;
+    if ( runAt === 0 ) {
+        setConstant(chain, cValue); return;
+    }
+    const docReadyState = ( ) => {
+        return ({ loading: 1, interactive: 2, complete: 3, })[document.readyState] || 0;
+    };
+    if ( docReadyState() >= runAt ) {
+        setConstant(chain, cValue); return;
+    }
+    const onReadyStateChange = ( ) => {
+        if ( docReadyState() < runAt ) { return; }
+        setConstant(chain, cValue);
+        document.removeEventListener('readystatechange', onReadyStateChange);
+    };
+    document.addEventListener('readystatechange', onReadyStateChange);
+}
 
 /******************************************************************************/