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Add policy on default-allowing C++ standards, and default-allow C++11.

https://groups.google.com/a/chromium.org/d/topic/cxx/gv7E_n4AnP4/discussion

Bug: none
Change-Id: I5d787e8965d5d6ed0d5037550405aba0375ba2ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1661211
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#669865}