DevTools: [MQI] add tooltips for ruler serifs

The patch adds tooltips with actual pixel values of serifs, representing
CSS breakpoints on CSS ruler.

BUG=391240
R=pfeldman, dgozman

Review URL: https://codereview.chromium.org/478953002

git-svn-id: svn://svn.chromium.org/blink/trunk@180411 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/devtools/front_end/toolbox/MediaQueryInspector.js

@@ -357,6 +357,7 @@ WebInspector.MediaQueryInspector.prototype = {
         var thresholds = this._mediaQueryThresholds();
         for (var i = 0; i < thresholds.length; ++i) {
             var thresholdElement = this._rulerDecorationLayer.createChild("div", "media-inspector-threshold-serif");
+            thresholdElement.title = thresholds[i] + "px";
             thresholdElement._value = thresholds[i];
             thresholdElement.style.left = (thresholds[i] - this._offset) / zoomFactor + "px";
         }