Get VS 2015 to use const int definitions

VC++ up to VS 2015 RTM does not require explicit storage allocation for
static const integers declared in classes. VS 2015 Update 1 requires
these storage definitions in some cases. It's unclear exactly what
cases - simple tests work with and without the explicit storage
allocation.

Many previous versions of VC++ have theoretically *allowed* a
definition to supply storage, but tests on VC++ 2013 show that this
doesn't actually work correctly - it leads to duplicate definition
errors. So, the change is scoped to VS 2015 only.

This changed was landed in the upstream protobuf repo:
https://github.com/google/protobuf/commit/a74e912a8be1274

With this change the chrome target builds with the latest VS 2015.

R=scottmg@chromium.org, pdr@chromium.org
BUG=440500

Review URL: https://codereview.chromium.org/1422453005

Cr-Commit-Position: refs/heads/master@{#357013}

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