Add a setting for keeping around all processes and always reusing them per-origin.

Add a setting for keeping around all processes and always reusing them per-origin.
<rdar://problem/39695798> and https://bugs.webkit.org/show_bug.cgi?id=185020

Reviewed by Andy Estes.

Source/WebKit:

* UIProcess/API/APIProcessPoolConfiguration.cpp:
(API::ProcessPoolConfiguration::copy):
* UIProcess/API/APIProcessPoolConfiguration.h:

* UIProcess/API/C/WKContextConfigurationRef.cpp:
(WKContextConfigurationAlwaysKeepAndReuseSwappedProcesses):
(WKContextConfigurationSetAlwaysKeepAndReuseSwappedProcesses):
* UIProcess/API/C/WKContextConfigurationRef.h:

* UIProcess/API/Cocoa/_WKProcessPoolConfiguration.h:
* UIProcess/API/Cocoa/_WKProcessPoolConfiguration.mm:
(-[_WKProcessPoolConfiguration setAlwaysKeepAndReuseSwappedProcesses:]):
(-[_WKProcessPoolConfiguration alwaysKeepAndReuseSwappedProcesses]):

* UIProcess/SuspendedPageProxy.cpp:
(WebKit::SuspendedPageProxy::webProcessDidClose):
(WebKit::SuspendedPageProxy::destroyWebPageInWebProcess):
* UIProcess/SuspendedPageProxy.h:

* UIProcess/WebPageProxy.cpp:
(WebKit::WebPageProxy::suspendedPageClosed):
(WebKit::WebPageProxy::suspendedPageProcessClosed): Deleted.
* UIProcess/WebPageProxy.h:
(WebKit::WebPageProxy::suspendedPage const):

* UIProcess/WebProcessPool.cpp:
(WebKit::WebProcessPool::shouldTerminate):
(WebKit::WebProcessPool::disconnectProcess):
(WebKit::WebProcessPool::addProcessToOriginCacheSet):
(WebKit::WebProcessPool::removeProcessFromOriginCacheSet):
(WebKit::WebProcessPool::processForNavigation): If a swap will occur, cache the old process.
(WebKit::WebProcessPool::processForNavigationInternal): Consider re-using a previously cached process.
* UIProcess/WebProcessPool.h:

* WebProcess/WebPage/WebPage.cpp:
(WebKit::m_credentialsMessenger):

Tools:

* TestWebKitAPI/Tests/WebKitCocoa/ProcessSwapOnNavigation.mm:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@231048 268f45cc-cd09-0410-ab3c-d52691b4dbfc