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Currently, GUIDs are returned as and stored in std::string. A dedicated
base::GUID class would introduce greater type safety.

Mark existing GUID functions as deprecated.

Add tests for the new class.

Bug: 1026195
Change-Id: I15d0c8a5907291d48d2e1ddf029390170e81993b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2200456
Commit-Queue: Daniel Hosseinian <dhoss@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827030}