Yes - I bet this is because we're no longer using the 'observers' on the backend. I'll have to manually notify the renderer when things are cleared by the backend. Hmm.....

I'm going to keep things as they are for a couple days while I develop a fix. If it takes longer than 2 days, then I'll revert the launch.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/cd0f56ea1726c5df4e08250bd6ea0c630c0252b8

commit cd0f56ea1726c5df4e08250bd6ea0c630c0252b8
Author: Daniel Murphy <dmurph@chromium.org>
Date: Tue Jun 26 23:01:07 2018

[SessionStorageS13N] Observer support for clearing site data

This uses the (previously unused by SessionStorage) StorageArea's
observer interface to notify the renderer when session storage is
cleared by the browser. This change
* Updates the service to notify observers in this way
* Adds tests in the service (& cleans up old observer usage)
* Modifies WebStorageNamespace to include a WebString namespace getter.
  This lets the IsSameNamespace comparision to not cause undefined
  behavior when comparing different subclasses of WebStorageNamespace.

R=mek@chromium.org

Bug:  856120 
Change-Id: I97e2d7a8f7a2b660143bf37bcbd04f5a3985bce2
Reviewed-on: https://chromium-review.googlesource.com/1114300
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570568}
[modify] https://crrev.com/cd0f56ea1726c5df4e08250bd6ea0c630c0252b8/content/browser/dom_storage/session_storage_area_impl.cc
[modify] https://crrev.com/cd0f56ea1726c5df4e08250bd6ea0c630c0252b8/content/browser/dom_storage/session_storage_area_impl.h
[modify] https://crrev.com/cd0f56ea1726c5df4e08250bd6ea0c630c0252b8/content/browser/dom_storage/session_storage_area_impl_unittest.cc
[modify] https://crrev.com/cd0f56ea1726c5df4e08250bd6ea0c630c0252b8/content/browser/dom_storage/session_storage_context_mojo_unittest.cc
[modify] https://crrev.com/cd0f56ea1726c5df4e08250bd6ea0c630c0252b8/content/browser/dom_storage/session_storage_namespace_impl_mojo.cc
[modify] https://crrev.com/cd0f56ea1726c5df4e08250bd6ea0c630c0252b8/content/browser/dom_storage/session_storage_namespace_impl_mojo_unittest.cc
[modify] https://crrev.com/cd0f56ea1726c5df4e08250bd6ea0c630c0252b8/content/renderer/dom_storage/local_storage_cached_area.cc
[modify] https://crrev.com/cd0f56ea1726c5df4e08250bd6ea0c630c0252b8/content/renderer/dom_storage/local_storage_namespace.cc
[modify] https://crrev.com/cd0f56ea1726c5df4e08250bd6ea0c630c0252b8/content/renderer/dom_storage/local_storage_namespace.h
[modify] https://crrev.com/cd0f56ea1726c5df4e08250bd6ea0c630c0252b8/content/renderer/dom_storage/session_web_storage_namespace_impl.cc
[modify] https://crrev.com/cd0f56ea1726c5df4e08250bd6ea0c630c0252b8/content/renderer/dom_storage/session_web_storage_namespace_impl.h
[modify] https://crrev.com/cd0f56ea1726c5df4e08250bd6ea0c630c0252b8/content/renderer/dom_storage/webstoragenamespace_impl.cc
[modify] https://crrev.com/cd0f56ea1726c5df4e08250bd6ea0c630c0252b8/content/renderer/dom_storage/webstoragenamespace_impl.h
[modify] https://crrev.com/cd0f56ea1726c5df4e08250bd6ea0c630c0252b8/third_party/blink/public/platform/web_storage_namespace.h