Firefox already has this implemented. WebKit bug for this is here:
https://bugs.webkit.org/show_bug.cgi?id=191643

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3248501c9931ea2061fa42291709ddd7be1625be

commit 3248501c9931ea2061fa42291709ddd7be1625be
Author: Rob Buis <rbuis@igalia.com>
Date: Mon Nov 19 20:06:09 2018

Setting document.title should have no effect for non SVG/HTML documents

Setting document.title should have no effect for non SVG/HTML documents,
see https://html.spec.whatwg.org/multipage/dom.html#document.title.

Behavior matches Firefox.

Bug:  906038 

Change-Id: I724abf9ecd711536d71ad95ab45c72230c2e87b6
Reviewed-on: https://chromium-review.googlesource.com/c/1339959
Commit-Queue: Rob Buis <rbuis@igalia.com>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609407}
[add] https://crrev.com/3248501c9931ea2061fa42291709ddd7be1625be/third_party/WebKit/LayoutTests/external/wpt/html/dom/documents/dom-tree-accessors/document.title-not-in-html-svg.html
[modify] https://crrev.com/3248501c9931ea2061fa42291709ddd7be1625be/third_party/blink/renderer/core/dom/document.cc