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Friend'ing specific methods is incorrect since the addition of
GetIfExits() because, although clang compiles this without warnings,
the spec clearly says that friend'ing an undeclared method is not
allowed (even if no such method ever exists and hence never uses that
friend declaration). Not nearly all singletons need to support
GetInstanceIfExists() and hence adding everywhere is undesired.

This CL makes Singleton<T> friend all of T. This is fine because
that doesn't give much more power to T (beyond the option of naming
its GetInstance() method differently...).

Bug: 905990
Change-Id: I2d2c721b27f8f3fa3367dff996f37293c98674c3
Reviewed-on: https://chromium-review.googlesource.com/c/1343660Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609809}