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This CL saves 27KB in chrome.dll and 5KB in chrome_child.dll.

https://chromium-review.googlesource.com/868013 had regressed size by
9KB and was caught as a performance regression. This CL should recover
this 3X.

Also removed an extra atomic operation in the simple case (used to
atomic-load for check and atomic-load again for return, now only
atomic-loads once).

Also moved GetOrCreateLazyPointer() to a subtle:: namespace (now that
calling it is tricky), also resolving recurring confusion about whether
Needs/CompleteLazyInstance() are invoked outside of it (they're
not, they're strictly helpers).

R=fdoray@chromium.org, thakis@chromium.org

Bug: 804034
Change-Id: I8de94fe742a8f9a68d4a66e0202a6fb276843af9
Reviewed-on: https://chromium-review.googlesource.com/883364Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532013}