DevTools: fix tooltip logic for elements within shadow DOM

Tooltips only show when the mouse target is visible/has offsetParent.
Elements using shadow DOM can have <content> elements in the event
path, which may be visible despite having no offsetParent.

Bug: 847387
Change-Id: Ie5dab99d27146787ad7b14bdf95c7932af1e712d
Reviewed-on: https://chromium-review.googlesource.com/1077328Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Erik Luo <luoe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563062}

third_party/blink/renderer/devtools/front_end/ui/Tooltip.js

@@ -63,7 +63,7 @@ UI.Tooltip = class {
 
     for (const element of path) {
       // The offsetParent is null when the element or an ancestor has 'display: none'.
-      if (element === this._anchorElement || element.offsetParent === null) {
+      if (element === this._anchorElement || (element.nodeName !== 'CONTENT' && element.offsetParent === null)) {
         return;
       } else if (element[UI.Tooltip._symbol]) {
         this._show(element, mouseEvent);