Following https://github.com/brave/brave-browser/issues/15546, we should also migrate BraveFrameMsg_AllowScriptsOnce to mojo (the only legacy IPC method remaining in Brave) which, besides modernizing the code, it also allows us to remove some bits that are no longer needed. This change is covered by the following browser tests: BraveShieldsAPIBrowserTest.AllowScriptsOnce BraveShieldsAPIBrowserTest.AllowScriptsOnceDataURL BraveShieldsAPIBrowserTest.AllowScriptsOnceIframe Resolves https://github.com/brave/brave-browser/issues/15573