Remove `friend class` from SerializedCookiePartitionKey

CookiePartitionKey just needs the ability to call this
particular ctor; it doesn't need any stronger capabilities
granted by the `friend class` statement. The blessed way to
give access to specific APIs to a certain caller (while
making those APIs unavailable to other callers) is to use
a base::PassKey.

Change-Id: Ia667b61f46b98c64e68c79b96c8f609f3ec372e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5708809
Commit-Queue: Chris Fredrickson <cfredric@chromium.org>
Reviewed-by: Dylan Cutler <dylancutler@google.com>
Auto-Submit: Chris Fredrickson <cfredric@chromium.org>
Commit-Queue: Dylan Cutler <dylancutler@google.com>
Cr-Commit-Position: refs/heads/main@{#1327646}