[devtools] Fix coverage gutter display
[devtools] Fix coverage gutter display

The element with the color collapsed to zero width,
because its only content was a whitespace character
that got collapsed. I replaced it with a zero-width
whitespace, this way the element gets the right width
from CSS.

Bug: chromium:996774
Change-Id: Ie6322196c7575b0c1b2c402b97bb2928f62ed114
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1774631
Reviewed-by: Yang Guo <1cf0af8245a3a17a632a2c73531392b0f9761c76@chromium.org>
Reviewed-by: Andrey Kosyakov <03b08926e49d80331ab322c2b9e0332109fbfd39@chromium.org>
Commit-Queue: Andrey Kosyakov <03b08926e49d80331ab322c2b9e0332109fbfd39@chromium.org>
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#691623}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: e5f87144df35ad4b18414681d414a12e9d715e6f

ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend