The Chrome extension enforces that local files cannot be embedded in
non-local web pages. The previous check was too strict (because the
origin of a file:-URL is "null"), and prevented local PDF from being
viewed in local files).
This patch fixes that problem, by querying the actual tab URL via the
background page.
Steps to verify:
1. Create a HTML file: `<iframe src=test.pdf width=100% height=100%>`
2. Build and load the extension.
3. Allow file access to the extension at `chrome://extensions`
4. Open the HTML file from a file:// URL.
5. VERIFY: The extension should attempt to load the PDF file.
6. Now open the following (replace ID with the extension ID, which you
can find at `chrome://extensions`):
`data:text/html,<iframe src="chrome-extension://ID/file:///test.pdf">`
7. VERIFY: The next error should be displayed:
"Refused to load a local file in a non-local page for security reasons."