Make use of `X-DNS-Prefetch-Control` in Chromium-based browsers

Related issue:
- https://github.com/uBlockOrigin/uBlock-issues/issues/548

The fix applies only to Chromium-based browsers -- a
`X-DNS-Prefetch-Control` header[1] will be unconditionally
injected when uBO's "Disable pre-fetching" setting is
enabled (it is by default).

This is a mitigation, this does not completely fix the issue
of the setting "Disable pre-fetching" being disregarded on
Chromium-based browsers when sites use
`preconnect`/`preload`.

[1] https://developer.mozilla.org/docs/Web/HTTP/Headers/X-DNS-Prefetch-Control

platform/chromium/vapi-background.js

@@ -223,8 +223,9 @@ vAPI.browserSettings = (function() {
                 }
                 switch ( setting ) {
                 case 'prefetching':
+                    const enabled = !!details[setting];
                     try {
-                        if ( !!details[setting] ) {
+                        if ( enabled ) {
                             chrome.privacy.network.networkPredictionEnabled.clear({
                                 scope: 'regular'
                             }, vAPI.resetLastError);
@@ -237,6 +238,9 @@ vAPI.browserSettings = (function() {
                     } catch(ex) {
                         console.error(ex);
                     }
+                    if ( vAPI.prefetching instanceof Function ) {
+                        vAPI.prefetching(enabled);
+                    }
                     break;
 
                 case 'hyperlinkAuditing':

platform/chromium/vapi-webrequest.js

@@ -195,3 +195,39 @@ vAPI.net.onBeforeReady = vAPI.net.onBeforeReady || (function() {
 })();
 
 /******************************************************************************/
+
+// https://github.com/uBlockOrigin/uBlock-issues/issues/548
+//   Use `X-DNS-Prefetch-Control` to workaround Chromium's disregard of the
+//   setting "Predict network actions to improve page load performance".
+
+vAPI.prefetching = (function() {
+    let listening = false;
+
+    const onHeadersReceived = function(details) {
+        details.responseHeaders.push({
+            name: 'X-DNS-Prefetch-Control',
+            value: 'off'
+        });
+        return { responseHeaders: details.responseHeaders };
+    };
+
+    return state => {
+        const wr = chrome.webRequest;
+        if ( state && listening ) {
+            wr.onHeadersReceived.removeListener(onHeadersReceived);
+            listening = false;
+        } else if ( !state && !listening ) {
+            wr.onHeadersReceived.addListener(
+                onHeadersReceived,
+                {
+                    urls: [ 'http://*/*', 'https://*/*' ],
+                    types: [ 'main_frame', 'sub_frame' ]
+                },
+                [ 'blocking', 'responseHeaders' ]
+            );
+            listening = true;
+        }
+    };
+})();
+
+/******************************************************************************/