Add missing PLATFORM_EXPORT annotations

This CL should fix the component build on Linux.

R=eae@chromium.org, eae
BUG=297477

Review URL: https://codereview.chromium.org/25586003

git-svn-id: svn://svn.chromium.org/blink/trunk@158735 bbb929c8-8fbe-4397-9dbb-9b2b20218538