Use shared ChildProcess code to enter plug-in sandbox.

        Reviewed by Sam Weinig.

        There is one known behavior change from this refactoring: getpwuid_r is used
        instead of NSHomeDirectory for home directory, mathcing other client processes.

        * PluginProcess/PluginProcess.cpp: (WebKit::PluginProcess::enterSandbox):
        * PluginProcess/PluginProcess.h:
        PluginProcess prevents ChildProcess attempt to enter the sandbox immediately on
        launch for now, because we don't have a sandbox profile directory path yet.
        It now keeps a copy of ChildProcessInitializationParameters, so that
        ChildProcess::initializeSandbox() could be called later.

        * PluginProcess/mac/PluginProcessMac.mm:
        (WebKit::PluginProcess::platformInitializeProcess): Store a copy of ChildProcessInitializationParameters
        for later.
        (WebKit::loadSandboxProfile): Build a sandbox profile from a common prefix and
        a plugin-specific part.
        (WebKit::PluginProcess::platformInitializePluginProcess): We can enter the sandbox now.
        (WebKit::PluginProcess::enterSandbox): Prepare SandboxInitializationParameters,
        and call ChildProcess::initializeSandbox().

        * Shared/mac/ChildProcessMac.mm:
        (WebKit::ChildProcess::initializeSandbox): Actually handle system directory suffix
        from parameters.

        * WebKit2.xcodeproj/project.pbxproj:
        * WebProcess/Plugins/Netscape/mac/NetscapeSandboxFunctions.h: Removed.
        * WebProcess/Plugins/Netscape/mac/NetscapeSandboxFunctions.mm: Removed.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@140926 268f45cc-cd09-0410-ab3c-d52691b4dbfc