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(1) Use relative paths in the downloads UI, instead of prefixing
everything with chrome://downloads.
(2) To make this work, add a <base href="chrome://downloads> to non
optimized builds.
(3) Match the behavior for incorrect subpaths like
chrome://downloads/a/b/ to the behavior for other Web UIs (settings,
extensions, and print preview).

Bug: none
Change-Id: I34978c736126537fddbb8118d03de0dce8201917
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1604054
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660018}