Use more URL constants throughout content_unittests.

This change eliminates several dependencies on hardcoded URL strings
thoughout test code in content_unittests. An already existing helper
function, GetWebUIURL, was moved into test_utils.h/.cc and taken
advantage of across several different test code files.

Bug: 955562
Change-Id: I3405e5f6c70d3f605a6bc9764cadbb68a6670ccb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1577851
Commit-Queue: Matt Siembor <msiembor@microsoft.com>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653760}
11 files changed