DevTools: introduce "Track CSS Usage" experiment

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

 .CodeMirror {
     line-height: 1.2em !important;
     background-color: transparent !important;
     color: #222;
 }
 
 .CodeMirror-linewidget {
     overflow: visible !important;
 }
 
 .CodeMirror-gutter-performance {
     width: 74px;
     background-color: white;
     margin-left: 3px;
 }
 
+.CodeMirror-gutter-cssTracking {

[caseq, 2016/11/04 21:10:02]

I don't think that CodeMirror should be concerned with CSS rule usage tracking.

+    width: 20px;
+    background-color: white;
+    margin-left: 3px;
+}
+
+.CodeMirror-gutter-cssTracking::after{
+    content: " ";
+}
+
 .CodeMirror .source-frame-eval-expression {
     outline: 0;
     border: 1px solid rgb(163, 41, 34);
     border-left-width: 0;
     border-right-width: 0;
     background-color: rgb(255, 255, 194);
 }
 
 .CodeMirror .source-frame-eval-expression-end {
     border-right-width: 1px;
@@ skipping 234 matching lines @@
 
 .-theme-with-dark-background .cm-line-with-selection .cm-search-highlight {
     color: #eee;
 }
 
 .CodeMirror .text-editor-line-marker-performance {
     text-align: right;
     padding-right: 3px;
 }
 
+.CodeMirror .text-editor-line-marker-cssTracking {
+    text-align: right;
+    padding-right: 3px;
+    background-color: red;
+}
+
+.CodeMirror .text-editor-line-marker-cssTracking::after{
+    content: " ";
+}
+
 .CodeMirror .text-editor-line-decoration {
     position: absolute;
 }
 
 .CodeMirror .text-editor-line-decoration-wave {
     position: absolute;
     top: -2px;
     right: -4px;
     left: 4px;
     cursor: pointer;
@@ skipping 90 matching lines @@
 }
 
 .CodeMirror .auto-complete-text {
     color: rgb(128,128,128);
 }
 
 /** Prevent the codemirror textarea from stealing PageUp events **/
 .CodeMirror textarea {
     resize: none;
     overflow: hidden;
-}
+}