This is needed for propagating the extension's permissions
to the extension's iframe, in the rare event that the PDF is
loaded in a sub frame, and the extension does not have access to the
top frame. For instance, when a http:-PDF file is embedded in a
local file, while "Allow access to local URLs" is disabled.
Note: Propagating permissions by inserting content scripts is an
undocumented feature (http://crbug.com/302548).
Whenever it breaks, the issue (cross-domain permissions for XHR)
can be solved by using a content script that gets the blob using
the XMLHttpRequest API, followed by `postMessage` (via transferables)
to efficiently pass the arraybuffer back to the PDF Viewer.
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