Screencast URL bar now shows decoded characters

Currently, the Screencast URL bar will show URLs in encoded form
when the URL contains characters that require escape sequences.
For example, this link to Korean Wikipedia:

https://ko.wikipedia.org/wiki/위키백과:대문

will appear as the encoded string in Screencast view:

https://ko.wikipedia.org/wiki/%EC%9C%84%ED%82%A4%EB%B0%B1%EA%B3%BC:%EC%A7%88%EB%AC%B8%EB%B0%A9

The Chrome omnibox always shows the URLs in decoded form
(user friendly), Korean characters included. Thus there is a mismatch
between what Chrome displays in its addressbar (decoded) and what
the Screencast view displays (encoded).

This CL updates URL bar in the Screencast view to display URLs in
decoded (user friendly) form, so it displays the same characters as
the Chrome address bar.

Note that the Chrome addressbar supports *both* encoded and
decoded forms of URLs as input, but will convert to decoded once
the user presses enter. This CL matches the behavior by allowing
the screencast URL bar to accept both forms while converting to
decoded once the user presses enter.

Before: https://i.imgur.com/crJ4S7P.gif
After: https://i.imgur.com/N7TsVXa.gif

Bug: 1070462
Change-Id: Ie7e11a7cda62cbc15bf2e8be26fa835076586893
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2166449
Reviewed-by: Brandon Goddard <brgoddar@microsoft.com>
Reviewed-by: Vidal Diazleal <vidorteg@microsoft.com>
Reviewed-by: Songtao Xia <soxia@microsoft.com>
Reviewed-by: Jack Lynch <jalyn@microsoft.com>
Commit-Queue: Brian Cui <brcui@microsoft.com>