Add |WebTextDirection toWebTextDirection(TextDirection)|.

We need to add "platform" to public/web/DEPS in order to include
platform/text/TextDirection.h in WebTextDirection.h.
This change should have no problems because we already have "core"
dependency, and "core" depends on "platform."

TEST=none; no behavior changes.
BUG=

Review URL: https://codereview.chromium.org/470883002

git-svn-id: svn://svn.chromium.org/blink/trunk@180316 bbb929c8-8fbe-4397-9dbb-9b2b20218538