[COOP] access reporting: Add body.initialPopupURL

The [coop-reporting-explainer] defined several URLs that might be
reported:
- initial_popup_url
- openee_url
- opener_url
- other_document_url
- referrer

This patch adds body.initialPopupURL

[coop-reporting-explainer]:
https://github.com/camillelamy/explainers/blob/master/coop_reporting.md

Bug: 1090273
Change-Id: I535e44e66578fc90f449be59586f19b154677caf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2424092
Commit-Queue: Arthur Sonzogni <[email protected]>
Reviewed-by: Camille Lamy <[email protected]>
Reviewed-by: Mike West <[email protected]>
Cr-Commit-Position: refs/heads/master@{#812588}
diff --git a/content/public/browser/web_contents.h b/content/public/browser/web_contents.h
index b879df2..0426cc15 100644
--- a/content/public/browser/web_contents.h
+++ b/content/public/browser/web_contents.h
@@ -151,6 +151,12 @@
     // window.open('', 'bar')).
     std::string main_frame_name;
 
+    // New window starts from the initial empty document. When created by an
+    // opener, the latter can request an initial navigation attempt to be made.
+    // This is the url specified in: `window.open(initial_popup_url, ...)`.
+    // This is empty otherwise.
+    GURL initial_popup_url;
+
     // True if the contents should be initially hidden.
     bool initially_hidden;