Expose Safe Browsing SPI

Expose Safe Browsing SPI
https://bugs.webkit.org/show_bug.cgi?id=181804
<rdar://problem/36626946>

Patch by Zach Li <zacharyli323@gmail.com> on 2018-01-19
Reviewed by Alex Christensen.

If client is using Apple internal SDK, then we can just import the header; if not,
then we declare necessary symbols that client will need.

* Configurations/WebKit.xcconfig:
Only link against SafariSafeBrowsing framework on macOS High Sierra
and iOS 11 and above. Weak link against SafariSafeBrowsing framework
because it is not present on the Base system.

* Platform/spi/Cocoa/SafeBrowsingSPI.h: Added.

* WebKit.xcodeproj/project.pbxproj:
Added SafeBrowsingSPI.h.

* WebKitLibraries/WebKitPrivateFrameworkStubs/iOS/11/SafariSafeBrowsing.framework/SafariSafeBrowsing.tbd:
Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@227211 268f45cc-cd09-0410-ab3c-d52691b4dbfc