gcc rejects the following snippet, clang rejects it in -std=c++11 mode:
  namespace A { template<class T> class C {}; }
  namespace B { template class A::C<int>; }

Indeed, the C++ standard says in 14.7.2p2 "An explicit instantiation shall
appear in an enclosing namespace of its template", so cl.exe is incorrect to
allow this.

Just move the instantiation out of the v8 namespace to fix.  No intended
behavior change.  Fixes building with clang-cl on Windows.

BUG=chromium:475643
LOG=N
TBR=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/1073903002

Cr-Commit-Position: refs/heads/master@{#27721}