Just a note that the current (in progress) implementation doesn't assign navigation ids until commit. The change notes:

  // TODO( crbug.com/792662 ): Assign these to navigations earlier and retrieve
  // them here.

https://chromium-review.googlesource.com/c/chromium/src/+/877398

we should keep this bug open after that change lands, so we continue to track that issue (or open a new issue specific to it).

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ed7ba25bf1ed9cb8ce2964c7fb046d2094d403b7

commit ed7ba25bf1ed9cb8ce2964c7fb046d2094d403b7
Author: Steven Holte <holte@chromium.org>
Date: Sat Feb 10 22:11:28 2018

Add NavigationId and IsDownload to NavigationContext

These will be required for implementing UKMs SourceUrlRecorder for iOS.

Bug:  792662 
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I110d51ddf79dd1e08e4759721387d21cf280c439
Reviewed-on: https://chromium-review.googlesource.com/887954
Commit-Queue: Steven Holte <holte@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#535987}
[modify] https://crrev.com/ed7ba25bf1ed9cb8ce2964c7fb046d2094d403b7/ios/web/public/test/fakes/fake_navigation_context.h
[modify] https://crrev.com/ed7ba25bf1ed9cb8ce2964c7fb046d2094d403b7/ios/web/public/test/fakes/fake_navigation_context.mm
[modify] https://crrev.com/ed7ba25bf1ed9cb8ce2964c7fb046d2094d403b7/ios/web/public/web_state/navigation_context.h
[modify] https://crrev.com/ed7ba25bf1ed9cb8ce2964c7fb046d2094d403b7/ios/web/web_state/navigation_and_load_callbacks_inttest.mm
[modify] https://crrev.com/ed7ba25bf1ed9cb8ce2964c7fb046d2094d403b7/ios/web/web_state/navigation_context_impl.h
[modify] https://crrev.com/ed7ba25bf1ed9cb8ce2964c7fb046d2094d403b7/ios/web/web_state/navigation_context_impl.mm
[modify] https://crrev.com/ed7ba25bf1ed9cb8ce2964c7fb046d2094d403b7/ios/web/web_state/navigation_context_impl_unittest.mm
[modify] https://crrev.com/ed7ba25bf1ed9cb8ce2964c7fb046d2094d403b7/ios/web/web_state/ui/crw_web_controller.mm

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/99efb451d533b01638941766f3bd4e08c62cc22f

commit 99efb451d533b01638941766f3bd4e08c62cc22f
Author: Steven Holte <holte@chromium.org>
Date: Tue Feb 13 22:31:00 2018

Add HasCommitted to NavigationContext.

This is needed for UKM to capture navigation URLs.

Bug:  792662 
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Iadd121f8fa0ff6cb94851c1c83a2063d2c82338b
Reviewed-on: https://chromium-review.googlesource.com/915015
Commit-Queue: Steven Holte <holte@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536516}
[modify] https://crrev.com/99efb451d533b01638941766f3bd4e08c62cc22f/ios/web/public/test/fakes/fake_navigation_context.h
[modify] https://crrev.com/99efb451d533b01638941766f3bd4e08c62cc22f/ios/web/public/test/fakes/fake_navigation_context.mm
[modify] https://crrev.com/99efb451d533b01638941766f3bd4e08c62cc22f/ios/web/public/web_state/navigation_context.h
[modify] https://crrev.com/99efb451d533b01638941766f3bd4e08c62cc22f/ios/web/web_state/navigation_and_load_callbacks_inttest.mm
[modify] https://crrev.com/99efb451d533b01638941766f3bd4e08c62cc22f/ios/web/web_state/navigation_context_impl.h
[modify] https://crrev.com/99efb451d533b01638941766f3bd4e08c62cc22f/ios/web/web_state/navigation_context_impl.mm
[modify] https://crrev.com/99efb451d533b01638941766f3bd4e08c62cc22f/ios/web/web_state/navigation_context_impl_unittest.mm
[modify] https://crrev.com/99efb451d533b01638941766f3bd4e08c62cc22f/ios/web/web_state/ui/crw_web_controller.mm

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9ee27abfd52a6fc4951be5010f1ed884823d7c12

commit 9ee27abfd52a6fc4951be5010f1ed884823d7c12
Author: Steven Holte <holte@chromium.org>
Date: Mon Feb 26 22:07:05 2018

UKM SourceUrlRecorder for iOS.

Records URLs of all NavigationContexts for URL whitelisting, and makes
last committed IDs available for WebState objects.

Bug:  792662 
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: If512086a73a893e8e0c4c49df28b4ef624764761
Reviewed-on: https://chromium-review.googlesource.com/877398
Commit-Queue: Steven Holte <holte@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539281}
[modify] https://crrev.com/9ee27abfd52a6fc4951be5010f1ed884823d7c12/ios/chrome/browser/history/history_tab_helper.mm
[modify] https://crrev.com/9ee27abfd52a6fc4951be5010f1ed884823d7c12/ios/chrome/browser/metrics/BUILD.gn
[add] https://crrev.com/9ee27abfd52a6fc4951be5010f1ed884823d7c12/ios/chrome/browser/metrics/ukm_url_recorder.h
[add] https://crrev.com/9ee27abfd52a6fc4951be5010f1ed884823d7c12/ios/chrome/browser/metrics/ukm_url_recorder.mm
[add] https://crrev.com/9ee27abfd52a6fc4951be5010f1ed884823d7c12/ios/chrome/browser/metrics/ukm_url_recorder_unittest.mm
[modify] https://crrev.com/9ee27abfd52a6fc4951be5010f1ed884823d7c12/ios/chrome/browser/tabs/tab_helper_util.mm
[modify] https://crrev.com/9ee27abfd52a6fc4951be5010f1ed884823d7c12/services/metrics/public/cpp/ukm_recorder.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5c275a501f5fc33ee560d35e47a4f315d496834c

commit 5c275a501f5fc33ee560d35e47a4f315d496834c
Author: Steven Holte <holte@google.com>
Date: Thu Oct 04 18:29:55 2018

Convert iOS PasswordManager UKMs to shared SourceId.

This matches the non-iOS behavior from:
https://chromium-review.googlesource.com/c/chromium/src/+/807347

BUG= 792662 

Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: I5f45059c4f874911d667187d48ea3848e62060cb
Reviewed-on: https://chromium-review.googlesource.com/c/1130472
Commit-Queue: Steven Holte <holte@chromium.org>
Reviewed-by: Vaclav Brozek <vabr@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596767}
[modify] https://crrev.com/5c275a501f5fc33ee560d35e47a4f315d496834c/ios/chrome/browser/metrics/BUILD.gn
[modify] https://crrev.com/5c275a501f5fc33ee560d35e47a4f315d496834c/ios/chrome/browser/passwords/BUILD.gn
[modify] https://crrev.com/5c275a501f5fc33ee560d35e47a4f315d496834c/ios/chrome/browser/passwords/ios_chrome_password_manager_client.h
[modify] https://crrev.com/5c275a501f5fc33ee560d35e47a4f315d496834c/ios/chrome/browser/passwords/ios_chrome_password_manager_client.mm
[modify] https://crrev.com/5c275a501f5fc33ee560d35e47a4f315d496834c/ios/chrome/browser/passwords/password_controller.mm