This is tripping some checks in  issue 718750 .

Step 1: Make Document::SetURL private. There are a horrifying number of calls to this function.
Step 2: Either add a new IPC or see if we can co-opt the existing same document navigation IPC.
 

When a document uses `document.write` on an about:blank page, it's supposed to alter the URL of the page written to. This correctly happens on the renderer side, but the browser side is never updated to match. It should be.

This caused DCHECKs in the dialog functions in  issue 718570 .

Step 1: Make Document::SetURL private. There are a horrifying number of calls to this function.
Step 2: Either add a new IPC or see if we can co-opt the existing same document navigation IPC.