Add API test for chrome.imageLoaderPrivate.getPdfThumbnail()

Currently, this API only has unit tests. Add an end-to-end
FileManagerPrivateApiTest to make sure it works in a full browser
environment. The C++ test runs 3 JS test cases to make sure the good
path and some easy to test bad paths work as expected. This ensures
code refactors in the near future that affect this API will not break
it.

To make the test work, change the imageLoaderPrivate API to not have
an allowlist on the "trunk" channel.

Bug: 332743737
Change-Id: Ia1dc2bffae1e3ca3d8bb36ca73432485bb4e5192
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5431014


Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1283472}