GitLab

This CL:
1. Stops null-checking the WebContents when considering to block popups.
   All callers pass in a valid contents already.

2. Tests the case when a caller calls ShowBlockedPopup with an invalid
   id. This seemed like a more reasonable API to be defensive with, but
   it wasn't tested.

Bug: None
Change-Id: I3913ed540b06f57e7fd31cae1f7da7de4da37dab
Reviewed-on: https://chromium-review.googlesource.com/1073847Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562022}