Fix SessionRestoration's active WebState

This CL fixes a typo in GetPinnedCount, which was reading the incorrect
key. It also fixes a test that was incorrect (and thus passed).

Fixed: 332672515
Change-Id: Ic239666053072414d77b96e2f5ed9621bbcf6d91
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5419317


Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Auto-Submit: Louis Romero <lpromero@google.com>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1281817}