Limit PDF helper extension to print preview only

These days, the helper extension is only used by the print preview.
So make sure that the helper extension is only activated in print
previews and fall back to the standard MimeHandler otherwise.
When the helper extension is unexpectedly loaded directly instead
of through the mime handler, the extension will fail to initialize.
This is totally desired!

TEST=See bug report, also confirmed that print preview still works.

Cr-Commit-Position: refs/heads/master@{#425280}
7 files changed