Shared library sharing of template-defined global variables is broken on
Windows. This prepares contextual variables to export their globals from
a DLL.

Change-Id: I3092a92db71c4e516218081b97335d451d90bfe2
Reviewed-on: https://chromium-review.googlesource.com/1148049Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54628}