DevTools: introduce "Track CSS Usage" experiment

third_party/WebKit/Source/devtools/front_end/text_editor/cmdevtools.css

Index: third_party/WebKit/Source/devtools/front_end/text_editor/cmdevtools.css
diff --git a/third_party/WebKit/Source/devtools/front_end/text_editor/cmdevtools.css b/third_party/WebKit/Source/devtools/front_end/text_editor/cmdevtools.css
index 4f7d1fcd436e6499fc05745867c05b77bbde5c5e..4a7ebd00ed40ff14e30ff961ddf770e6d0eadb22 100644
--- a/third_party/WebKit/Source/devtools/front_end/text_editor/cmdevtools.css
+++ b/third_party/WebKit/Source/devtools/front_end/text_editor/cmdevtools.css
@@ -14,6 +14,12 @@
     margin-left: 3px;
 }
 
+.CodeMirror-gutter-coverage {
+    width: 5px;
+    background-color: white;
+    margin-left: 3px;
+}
+
 .CodeMirror .source-frame-eval-expression {
     outline: 0;
     border: 1px solid rgb(163, 41, 34);
@@ -268,6 +274,16 @@ div.CodeMirror span.CodeMirror-nonmatchingbracket {
     padding-right: 3px;
 }
 
+.CodeMirror .text-editor-line-marker-coverage {
+    text-align: right;
+    padding-right: 2px;
+    background-color: darkred;
+}
+
+.CodeMirror .text-editor-line-marker-coverage::after {
+    content: " ";
+}
+
 .CodeMirror .text-editor-line-decoration {
     position: absolute;
 }