I think the concept of user gesture is content-specific and just piped to the UI from the renderer. We should probably just use an additional trigger for whether to open pdfviewer by looking at whether the URL transition is TYPED (which means it came from the omnibox).

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a06924c6c0170d0007966d0d86a3070a6ca0e717

commit a06924c6c0170d0007966d0d86a3070a6ca0e717
Author: qinmin <qinmin@chromium.org>
Date: Thu Jun 23 19:08:32 2016

Fix an issue that pdf is not auto-opened after typing url in omnibox

If a pdf download is triggered from omnibox, we don't auto-open the
file.
This is because the download item doesn't carry the user gesture flag.
However, in most cases the url is typed or the navigation is triggerd by
user.
This CL fixes the issue by identifying these downloads that are initiated
by user.

BUG= 622378 

Review-Url: https://codereview.chromium.org/2090853002
Cr-Commit-Position: refs/heads/master@{#401672}

[modify] https://crrev.com/a06924c6c0170d0007966d0d86a3070a6ca0e717/chrome/browser/android/download/download_controller.cc