Unreviewed trivial Linux buildfix after r164905.

setShouldCloseConnectionOnMachExceptions() is defined in the Mac only
Source/WebKit2/Platform/IPC/mac/ConnectionMac.cpp inside !PLATFORM(IOS)
which means PLATFORM(MAC), because nobody else uses this source file.

But ChildProcessProxy.cpp is used by Linux ports too, so we have to
use the most specific #if guard here, which is PLATFORM(MAC).

* Shared/ChildProcessProxy.cpp:
(WebKit::ChildProcessProxy::didFinishLaunching):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@164912 268f45cc-cd09-0410-ab3c-d52691b4dbfc