Organize all ClearSessionOnlyOrigins functions the same

There are 4 different ClearSessionsOnlyOrigins functions in
content/browser and 2 of them collided in jumbo build experiments.

This makes all 4 members of their logical class:
AppCacheStorageImpl::ClearSessionsOnlyOrigins
DOMStorageContextImpl::ClearSessionOnlyOrigins
IndexedDBContextImpl::ClearSessionOnlyOrigins
ServiceWorkerStorage::ClearSessionOnlyOrigins

Two of them (AppCache and IndexedDB) had to move for that to be
accomplished.

Bug: 746953
Change-Id: I9c8931b909fc78292e892dcff07d213abde2d8d1
Reviewed-on: https://chromium-review.googlesource.com/718753
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#508935}