Debug mechanism to override process QoS on Mac

https://bugs.webkit.org/show_bug.cgi?id=127455

Reviewed by Anders Carlsson.

* NetworkProcess/NetworkProcess.messages.in:
    - added message.
* PluginProcess/PluginProcess.messages.in:
    - added message.
* Shared/ChildProcess.h:
* Shared/mac/ChildProcessMac.mm:
(WebKit::ChildProcess::setQOS):
    - Added implementation of new message.
* UIProcess/Plugins/PluginProcessProxy.cpp:
(WebKit::PluginProcessProxy::didFinishLaunching):
    - SetQOS for the PluginProcess.
* UIProcess/Plugins/PluginProcessProxy.h:
* UIProcess/Plugins/mac/PluginProcessProxyMac.mm:
(WebKit::pluginProcessLatencyQOS):
(WebKit::pluginProcessThroughputQOS):
    - read NSUserDefaults.
* UIProcess/WebContext.cpp:
(WebKit::WebContext::ensureNetworkProcess):
    - SetQOS for the NetworkProcess.
(WebKit::WebContext::createNewWebProcess):
    - SetQOS for the WebProcess.
* UIProcess/WebContext.h:
* UIProcess/mac/WebContextMac.mm:
(WebKit::networkProcessLatencyQOS):
(WebKit::networkProcessThroughputQOS):
(WebKit::webProcessLatencyQOS):
(WebKit::webProcessThroughputQOS):
    - read NSUserDefaults.
* WebProcess/WebProcess.messages.in:
    - added message.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@162578 268f45cc-cd09-0410-ab3c-d52691b4dbfc