Move POD data structures into platform

This CL moves the POD data structures from core/platform to platform. It also
moves the helper code for their unit tests to platform/testing, letting us
delete the now-empty core/tests.

R=eseidel
BUG=297477

Review URL: https://codereview.chromium.org/26227002

git-svn-id: svn://svn.chromium.org/blink/trunk@159049 bbb929c8-8fbe-4397-9dbb-9b2b20218538