PDFPlugin: Enable PDFPlugin only if its dependencies exist on the system
https://bugs.webkit.org/show_bug.cgi?id=113372
<rdar://problem/12685301>

Reviewed by Alexey Proskuryakov.

* Shared/mac/PDFKitImports.h: Wrap in #if PLATFORM(MAC).
* WebProcess/WebPage/WebPage.cpp:
(WebKit::WebPage::createPlugin): Use shouldUsePDFPlugin.
(WebKit::WebPage::shouldUsePDFPlugin): If the PDFPlugin setting is enabled but PDFLayerController is not available, don't use PDFPlugin.
* WebProcess/WebPage/WebPage.h:
(WebPage): Add shouldUsePDFPlugin
* WebProcess/WebPage/mac/WebPageMac.mm:
(WebKit::WebPage::platformPreferencesDidChange): Use shouldUsePDFPlugin.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@147067 268f45cc-cd09-0410-ab3c-d52691b4dbfc