Chromium change:

https://chromium.googlesource.com/chromium/src/+/b6042ed37a482983111cdf1f9d0b09ffe972c309

commit b6042ed37a482983111cdf1f9d0b09ffe972c309
Author: Clark DuVall <cduvall@chromium.org>
Date:   Mon Jul 15 17:20:45 2019 +0000

    Support HandleExternalProtocol on UI thread

    This allows HandleExternalProtocol to be used on the UI thread in
    android_webview. This is part of migrating the navigation loader to the
    UI thread.

    Bug: 824840