fix: use appropriate site instance for cross-site nav's (#15821)

* fix: use Chromium's determined new site instance as candidate when navigating.

When navigating to a new address, consider using Chromium's determined site instance
for the new page as it should belong to an existing browsing instance when the
navigation was triggered by window.open().

fixes 8100.

* Revert "fix: use Chromium's determined new site instance as candidate when navigating."

This reverts commit eb95f935654a2c4d4457821297670836c10fdfd5.

* fix: delegate site instance creation back to content when sandboxed.

* fix: ensure site isolation is on

* test: adapt ut for cross-site navigation

* fix: register pending processes during a navigation.

* refactor: dont call loadURL for a window constructed from an existing webContents.

* test: add sandboxed affinity UT's.

* fix: check affinity before deciding if to force a new site instance.

* chore: adapt subsequent patch.

* refactor: constify logically const methods.

* fix: do not reuse site instances when navigation redirects cross-site.

* test: ensure localStorage accessible after x-site redirect.

* test: adapt localStorage acess denied UT for site isolation.

* fix: do not send render-view-deleted for speculative frames.

* chore: amend tests after rebase.

* test: add ut for webContents' render-view-deleted emission

* fix: introduce current-render-view-deleted for current RVH's deletions.

Revert render-view-deleted to being emitted with any RVH's deletion.
current-render-view-deleted is emitted only when the RVH being deleted
is the current one.

* refactor: style and comments fixed.

spec/fixtures/api/sandbox.html

@@ -81,6 +81,9 @@
       },
       'window-open-external': () => {
         addEventListener('load', () => {
+          ipcRenderer.once('open-the-popup', (event, url) => {
+            popup = open(url, '', 'top=65,left=55,width=505,height=605')
+          })
           ipcRenderer.once('touch-the-popup', () => {
             let errorMessage = null
             try {
@@ -90,7 +93,7 @@
             }
             ipcRenderer.send('answer', errorMessage)
           })
-          popup = open('http://www.google.com/#q=electron', '', 'top=65,left=55,width=505,height=605')
+          ipcRenderer.send('opener-loaded')
         })
       },
       'verify-ipc-sender': () => {