The portal document currently isn't being collected after the <portal> element is GC'd. This causes any layout test that has a cross-origin portal to fail with --enable-leak-detection.

Example failure: https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/WebKit%20Linux%20Trusty%20Leak/29014 

^ The CL (https://crrev.com/c/1399569) that landed the failing test got reverted.
 

If we embed a portal with origin B, in a main-frame with origin A, and embed an iframe with origin A inside the portal, the iframe document leaks when the portal is removed.

Example failure: https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/WebKit%20Linux%20Trusty%20Leak/29014 

^ The CL (https://crrev.com/c/1399569) that landed the failing test got reverted.