[devtools] Support for urlencoding in workspaces

This patch adds decodes urls before trying to associate them
with files from the local filesystem in workspaces.

Bug: chromium:992717
Change-Id: I57678ee5db7fab7c43a254f9440b25744356c3c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1807503
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jan Scheffler <janscheffler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698851}