DevTools: fix tooltip logic for elements within shadow DOM
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/1077328
Reviewed-by: Dmitry Gozman <0ca14b32cbb359a30ac562af27dd84cc2813eacb@chromium.org>
Commit-Queue: Erik Luo <26192f5e05d23e5ee959014fb911d780e566545b@chromium.org>
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#563062}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 48044f127f5994b33fcec580a15cc46664c8e54f
ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,dart-archive/devtools-frontend,dart-archive/devtools-frontend,dart-archive/devtools-frontend,dart-archive/devtools-frontend,ChromeDevTools/devtools-frontend,dart-archive/devtools-frontend,ChromeDevTools/devtools-frontend,dart-archive/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend