…e code

Given that only the `FileSpec.prototype.serializable` getter is ever invoked from "outside" of the class, and only once per `FileSpec`-instance, the caching/shadowing isn't actually necessary.

Furthermore the `_contentRef`-caching wasn't actually correct, since it ended up storing a `BaseStream`-instance and those should *generally* never be cached.
(Since calling `BaseStream.prototype.getBytes()` more than once, without resetting the stream in between, will return an empty TypedArray after the first time.)

(During rebasing of the previous patches I happened to look at the polyfills and noticed that this one could be removed now.)

Note:
 - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/try#browser_compatibility
 - https://bugzilla.mozilla.org/show_bug.cgi?id=1928493

…se64()`, and `Uint8Array.fromBase64()` polyfills

(During rebasing of the previous patches I happened to look at the polyfills and noticed that this one could be removed now.)

See:
 - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array/toHex#browser_compatibility
 - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array/toBase64#browser_compatibility
 - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array/fromBase64#browser_compatibility

Note that technically this functionality can still be disabled via a preference in Firefox, however that's slated for removal in [bug 1985120](https://bugzilla.mozilla.org/show_bug.cgi?id=1985120).
Looking at the Firefox source-code, see https://searchfox.org/firefox-main/search?q=array.tobase64%28%29&path=&case=false&regexp=false, you can see that it's already being used *unconditionally* elsewhere in the browser hence removing the polyfills ought to be fine (since toggling the preference would break other parts of the browser).