Make SidePanelRegistry explicitly owned.

This CL is a refactor with no intended behavior change.

Prior to this CL, SidePanelRegistry inherited from SupportsUserData and
was attached to both Browser and WebContents. After this CL,
SidePanelCoordinator owns the window-scoped instance of the registry,
and TabFeatures owns the tab-scoped instance of the registry.

This CL makes the following minor changes on top:
* The tab-scoped instance of ExtensionSidePanelManager is instantiated
  from TabFeatures rather than TabHelpers.
* ExtensionSidePanelManager had methods named GetOrCreate*. They have
  been split into separate methods to Create and Get.
* Some logic in SidePanelCoordinator previously relied on static getters
  returning nullptr during early startup/teardown. These edge cases have
  been called out explicitly, as the new instance-scoped getters always
  return non-nullptr.
* SidePanelRegistry::Get was renamed to
  SidePanelRegistry::GetDeprecated. Not all use cases could be converted
  since many of the callsites do not yet use the Tab primitive.
* The per-tab SidePanelRegistry is passed directly to per-tab
  controllers.
* Two failing side search tests were deleted as the feature is dead but
  not yet removed.

Change-Id: Ieb9473e65f20168a4f626eb8a6de01166c444e1c
Bug: 345499793
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5612905


Commit-Queue: Erik Chen <erikchen@chromium.org>
Reviewed-by: Caroline Rising <corising@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1338103}