skirt std::chrono on MSAN builds

I don't know why, but only std::chrono's calls to clock_gettime() seem
to be affected by this MSAN bug.  Other calls into libc++ that call
libc, like std::to_string(int) calling snprintf, work fine.

CQ_INCLUDE_TRYBOTS=skia.primary:Test-Ubuntu-Clang-GCE-CPU-AVX2-x86_64-Debug-MSAN,Perf-Ubuntu-Clang-GCE-CPU-AVX2-x86_64-Debug-MSAN

BUG=skia:6504

Change-Id: I73fbe8793d2b5b5cca46ed68fb078a77d8748127
Reviewed-on: https://skia-review.googlesource.com/13502
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
3 files changed