Move LayoutUnit from core/platform/ to platform/

Move the LayoutUnit class and the LayoutUnit unit tests from the
the core/platform directory into the separate platform module.

BUG=297477
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/25401003

git-svn-id: svn://svn.chromium.org/blink/trunk@158644 bbb929c8-8fbe-4397-9dbb-9b2b20218538