Master frame selection for amp-ad 3p vendors

### Description

## Expected Behavior
One of the frames should be selected as master frame
## Actual Behavor
All the frames are selected as master frames in chrome v91

https://github.com/ampproject/amphtml/blob/main/3p/ampcontext-integration.js#L39 throws a security error even if the frame of the passed name is present in chrome v91. This causes every frame to become master frame

### Reproduction Steps

1. Navigate to any page using 3p amp-ad vendors
2. All the iframes are master frame

### Relevant Logs

```shell
Tested on Chrome Version 91.0.4472.114 and Version Version 91.0.4472.120
```

### Browser(s) Affected

Chrome

### OS(s) Affected

_No response_

### Device(s) Affected

_No response_

### AMP Version Affected

_No response_

Start at 3p/ampcontext-integration.js around line 39 and reproduce the issue with a 3p amp-ad vendor in Chrome 91. Investigate why the frame lookup raises a security error when the named frame exists. Done means one frame is selected as the master frame rather than every iframe.

Written by the indexing model from the issue text.