Do let grow subframe dictionary grow unbound

Related discussion:
- https://bugzilla.mozilla.org/show_bug.cgi?id=1652925

It's not clear the code here will fix the reported
issue, but I did identify that the subframe
dictionary of a very long-lived web page can
theoretically grow unbound.

platform/chromium/webext.js

@@ -101,6 +101,7 @@ const webext = {
     // https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/webNavigation
     webNavigation: {
         getFrame: promisify(chrome.webNavigation, 'getFrame'),
+        getAllFrames: promisify(chrome.webNavigation, 'getAllFrames'),
     },
     // https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/windows
     windows: {

src/js/pagestore.js

@@ -163,6 +163,7 @@ const FrameStore = class {
     }
 
     init(frameURL) {
+        this.t0 = Date.now();
         this.exceptCname = undefined;
         this.rawURL = frameURL;
         if ( frameURL !== undefined ) {
@@ -252,6 +253,7 @@ const PageStore = class {
 
         this.frames = new Map();
         this.setFrame(0, tabContext.rawURL);
+        this.frameAddCount = 0;
 
         // The current filtering context is cloned because:
         // - We may be called with or without the current context having been
@@ -359,8 +361,39 @@ const PageStore = class {
         const frameStore = this.frames.get(frameId);
         if ( frameStore !== undefined ) {
             frameStore.init(frameURL);
-        } else {
-            this.frames.set(frameId, FrameStore.factory(frameURL));
+            return;
+        }
+        this.frames.set(frameId, FrameStore.factory(frameURL));
+        this.frameAddCount += 1;
+        if ( (this.frameAddCount & 0b111111) !== 0 ) { return; }
+        this.pruneFrames();
+    }
+
+    // There is no event to tell us a specific subframe has been removed from
+    // the main document. The code below will remove subframes which are no
+    // longer present in the root document. Removing obsolete subframes is
+    // not a critical task, so this is executed just once on a while, to avoid
+    // bloated dictionary of subframes.
+    // A TTL is used to avoid race conditions when new iframes are added
+    // through the webRequest API but still not yet visible through the
+    // webNavigation API.
+    async pruneFrames() {
+        let entries;
+        try {
+            entries = await webext.webNavigation.getAllFrames({
+                tabId: this.tabId
+            });
+        } catch(ex) {
+        }
+        if ( Array.isArray(entries) === false ) { return; }
+        const toKeep = new Set();
+        for ( const { frameId } of entries ) {
+            toKeep.add(frameId);
+        }
+        const obsolete = Date.now() - 60000;
+        for ( const [ frameId, { t0 } ] of this.frames ) {
+            if ( toKeep.has(frameId) || t0 >= obsolete ) { continue; }
+            this.frames.delete(frameId);
         }
     }