pdf: Hide bookmark icon when no bookmarks

The change to display: inline-block in [1] was preventing
the hidden attribute from taking effect.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/1508014

Bug: 939695
Change-Id: Ifb76787827062b478a36615ff0146697d078a7c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1511101
Auto-Submit: dstockwell <dstockwell@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#639750}