For code that's part of the core library, rather than e.g. the `web/`-folder, we should always be careful about *directly* accessing any DOM methods.
The `navigator` is one such structure, which shouldn't be assumed to always be available and we should thus check that it's actually present.[1]
Hence this patch re-factors the `navigator.platform` access, in the `AnnotationLayer`-code, to ensure that it's generally safe. Furthermore, to reduce unnecessary repeated string-matching to determine the current platform, we're now using a shadowed getter which is evaluated only once instead (at first access).
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[1] Note e.g. the `isSyncFontLoadingSupported` getter, in the `src/display/font_loader.js` file.
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