Simplify mojo::internal::ArrayTraits.

Simplify mojo::internal::ArrayTraits.

Also, stop doing a whole bunch of unnecessary stuff in the move-only
case (some of it was necessary to handle move-only types, pre-C++11;
some of it -- like the alignment stuff -- was of rather dubious
necessity and correctness).

Probably it can/should be improved even more. E.g., PushBack() should be
reworked (into two overloaded versions, a const T& one and a T&& one).
That leaves only Clone(), which is a bit more tricky.

R=jamesr@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/745b55da533a4f73db721f4f78bcdd0070c39f5d

[viettrungluu, 2015-10-20 23:28:57]

(It builds and the tests pass, so it must be good, right?)

[jamesr, 2015-10-21 00:46:17]

https://build.chromium.org/p/tryserver.client.mojo/builders/Mojo%20Linux%20Tr...
failed something, but I don't think it's related

lgtm

[viettrungluu, 2015-10-21 02:06:52]

Committed patchset #1 (id:1) manually as
745b55da533a4f73db721f4f78bcdd0070c39f5d (presubmit successful).