PDFPlugin: Build PDFPlugin everywhere, enable at runtime
https://bugs.webkit.org/show_bug.cgi?id=107117

Reviewed by Alexey Proskuryakov.

Since PDFLayerController SPI is all forward-declared, the plugin should build
on all Mac platforms, and can be enabled at runtime.

* Configurations/FeatureDefines.xcconfig:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@140394 268f45cc-cd09-0410-ab3c-d52691b4dbfc