[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/+/1774631Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691623}

third_party/blink/renderer/devtools/front_end/text_editor/cmdevtools.css

@@ -441,7 +441,7 @@ div.CodeMirror:focus-within span.CodeMirror-nonmatchingbracket {
 }
 
 .CodeMirror .text-editor-coverage-unused-marker::after {
-    content: " ";
+    content: "\200B";
 }
 
 .CodeMirror .text-editor-coverage-used-marker {
@@ -451,7 +451,7 @@ div.CodeMirror:focus-within span.CodeMirror-nonmatchingbracket {
 }
 
 .CodeMirror .text-editor-coverage-used-marker::after {
-    content: " ";
+    content: "\200B";
 }
 
 .CodeMirror .text-editor-line-decoration {