Per https://groups.google.com/a/chromium.org/d/topic/platform-architecture-dev/wuLAMcSvVqs/discussion STL types and GURL are allowed at the boundary between Blink and Chromium.

Boundary code that needs this provision likely also needs to be able to convert between KURL / WebURL and GURL. For example,
third_party/blink/renderer/platform/exported/web_canonical_cookie.cc essentially duplicates WebURL::operator GURL().

This bug tracks figuring out how to expose the needed conversions to the code inside Blink (built with INSIDE_BLINK defined) that lives at the boundary between Blink and Chromium, and then removing the conversion code that will be made redundant by the solution, like ToGURL() in third_party/blink/renderer/platform/exported/web_canonical_cookie.cc