Enforce cache_path requirements for NetworkService (see issue #2622).

This change adds a new CefSettings.root_cache_path value that must be either
equal to or a parent directory of all CefSettings.cache_path and
CefRequestContextSettings.cache_path values. The sandbox may block read/write
access from the NetworkService to directories that do not meet this requirement.

To test: Run cefclient with a combination of the following flags:

--cache-path=c:\temp\cache
  Cache data should be persisted to the specified directory.

--request-context-per-browser
  A separate numbered cache directory should be created underneath the
  cache-path directory for each new browser instance.

--enable-network-service --disable-extensions
  Same tests, but with NetworkService enabled.

Known issues:
- When NetworkService is enabled a C:\temp\cache\cache\Cache directory is
  created (should be C:\temp\cache\Cache).

tests/cefclient/browser/root_window_manager.cc

@@ -12,6 +12,7 @@
 #include "tests/cefclient/browser/main_context.h"
 #include "tests/cefclient/browser/test_runner.h"
 #include "tests/shared/browser/extension_util.h"
+#include "tests/shared/browser/file_util.h"
 #include "tests/shared/browser/resource_util.h"
 #include "tests/shared/common/client_switches.h"
 
@@ -335,7 +336,7 @@ CefRefPtr<CefRequestContext> RootWindowManager::GetRequestContext(
         // isolated context objects.
         std::stringstream ss;
         ss << command_line->GetSwitchValue(switches::kCachePath).ToString()
-           << time(NULL);
+           << file_util::kPathSep << time(NULL);
         CefString(&settings.cache_path) = ss.str();
       }
     }