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It is time to remove the use of the term "session" in
//ios/chrome/browser/snapshots.

The identifier used to store and retrieve images from SnapshotCache
is called |snapshotID|. The SnapshotCache has no concept of a Tab
or WebState. It is merely a collection of images with identifiers.

However, SnapshotTabHelper and SnapshotGenerator are tied to a
WebState. Within those classes, the unique identifier for a WebState
is called |tabID|. When those classes call SnapshotCache, they use
the WebState's |tabID| as the identifier to be used in SnapshotCache.

Change-Id: Ic08cd0d70b66e4d699bf06c8b5d8afc093e90255
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2354807
Commit-Queue: edchin <edchin@chromium.org>
Reviewed-by: Mohammad Refaat <mrefaat@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799782}