WebUI: Use relative paths where possible in shared UI elements, part 1.

This not only results to smaller file size, but also makes the code more
portable, since it is less dependent on chrome:// URLs, and can eventually
lead to running those elements and tests outside the context of a browser_tests
and without js2gtest.

For example this makes it easier to keep maintaining the following WebUI demo
https://freshp86.github.io/chromium-webui/demo.html, and potentially move it within
the Chromium repo itself.

Bug: 866236
Change-Id: I1459789177ba5ede252c3ade3e0952686f4f01b5
Reviewed-on: https://chromium-review.googlesource.com/1144610
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578132}
42 files changed