core: suppress protocol timeout for app manifest bug in LR (#7184)

lighthouse-core/gather/driver.js

@@ -400,7 +400,20 @@ class Driver {
    */
   async getAppManifest() {
     this.setNextProtocolTimeout(3000);
-    const response = await this.sendCommand('Page.getAppManifest');
+    let response;
+    try {
+      response = await this.sendCommand('Page.getAppManifest');
+    } catch (err) {
+      if (err.code === 'PROTOCOL_TIMEOUT') {
+        // LR will timeout fetching the app manifest in some cases, move on without one.
+        // https://github.com/GoogleChrome/lighthouse/issues/7147#issuecomment-461210921
+        log.error('Driver', 'Failed fetching manifest', err);
+        return null;
+      }
+
+      throw err;
+    }
+
     let data = response.data;
 
     // We're not reading `response.errors` however it may contain critical and noncritical