Shared Storage: Update shared shared observer methods' names

Now that content::protocol::StorageHandler directly implements
content::SharedStorageRuntimeManager::SharedStorageObserverInterface,
and because StorageHandler also implements two other observer classes,
several of the interface method names ought to be more specific, to
make it clear that these methods are related to Shared Storage.

We correct this.

Bug: 401011862
Change-Id: I97976fc7ce49d7a207a368d291d41ed2a12c710b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6517473
Reviewed-by: Yao Xiao <yaoxia@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Cammie Smith Barnes <cammie@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1457644}