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).

libcef/browser/prefs/browser_prefs.cc

@@ -193,6 +193,12 @@ std::unique_ptr<PrefService> CreatePrefService(Profile* profile,
     // Based on ProfileImpl::RegisterProfilePrefs.
     registry->RegisterBooleanPref(prefs::kPrintingEnabled, true);
 
+    // Cache preferences.
+    // Based on ProfileImpl::RegisterProfilePrefs.
+    registry->RegisterFilePathPref(prefs::kDiskCacheDir, cache_path);
+    registry->RegisterIntegerPref(prefs::kDiskCacheSize, 0);
+    registry->RegisterIntegerPref(prefs::kMediaCacheSize, 0);
+
     // Spell checking preferences.
     // Modify defaults from SpellcheckServiceFactory::RegisterProfilePrefs.
     std::string spellcheck_lang =