Export effects classes with SK_API.

Export effects classes with SK_API.

So this works in shared_library build in chromium.

This issue was raised up in the following chromium-dev thread:
https://groups.google.com/a/chromium.org/forum/?hl=en&fromgroups=#!topic/chromium-dev/Z6qzEw8MwVY

R=epoger@google.com,tomhudson@google.com

Review URL: https://codereview.appspot.com/6601065

git-svn-id: http://skia.googlecode.com/svn/trunk@5914 2bbb7eff-a529-9590-31e7-b0007b416f81