Note that the lower_underscore style is specified by the style guide for cheap methods: http://google.github.io/styleguide/cppguide.html#Function_Names
Though since that method is virtual now it does seem a bit off.

You're right. It was reasonable the way it was implemented at first. However, as you mentioned, since that method is virtual now, there's no guarantee that the implementation would be cheap. The CL is out for review, if you'd like to do a quick LGTM: https://codereview.chromium.org/1865763002/ :)
Nathan has LGTM'd in spirit already.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/169a50b0ce067461b2716908e10cf02610c34825

commit 169a50b0ce067461b2716908e10cf02610c34825
Author: vakh <vakh@chromium.org>
Date: Thu Apr 07 22:14:24 2016

Rename download_protection_enabled to IsDownloadProtectionEnabled

BUG= 601094 

Review URL: https://codereview.chromium.org/1865763002

Cr-Commit-Position: refs/heads/master@{#385897}

[modify] https://crrev.com/169a50b0ce067461b2716908e10cf02610c34825/chrome/browser/safe_browsing/local_database_manager.cc
[modify] https://crrev.com/169a50b0ce067461b2716908e10cf02610c34825/chrome/browser/safe_browsing/local_database_manager.h
[modify] https://crrev.com/169a50b0ce067461b2716908e10cf02610c34825/chrome/browser/safe_browsing/safe_browsing_service.cc
[modify] https://crrev.com/169a50b0ce067461b2716908e10cf02610c34825/components/safe_browsing_db/database_manager.h
[modify] https://crrev.com/169a50b0ce067461b2716908e10cf02610c34825/components/safe_browsing_db/remote_database_manager.cc
[modify] https://crrev.com/169a50b0ce067461b2716908e10cf02610c34825/components/safe_browsing_db/remote_database_manager.h
[modify] https://crrev.com/169a50b0ce067461b2716908e10cf02610c34825/components/safe_browsing_db/test_database_manager.cc
[modify] https://crrev.com/169a50b0ce067461b2716908e10cf02610c34825/components/safe_browsing_db/test_database_manager.h