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To simplify the API of SessionServiceIOS in preparation of next
refactoring, change it to always require a session identifier.
Passing an empty string or nil will result in the same code path
as calling the methods that took no session identifier.

Bug: 1165798
Change-Id: I9739baf91cbcfcd21be61cd487c24b0d375289bb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2632760
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Stepan Khapugin <stkhapugin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845664}