[DevTools] Fix request preview for URI encoded html

Bug: 895081
Change-Id: I49c6131e1cc432e470e4b04353282d3ebebcb063
Reviewed-on: https://chromium-review.googlesource.com/c/1286758
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Joey Arhar <jarhar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601792}