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/context.h

@@ -73,10 +73,13 @@ class CefContext {
 
   CefTraceSubscriber* GetTraceSubscriber();
 
-  // Populate the request context settings based on CefSettings and command-
-  // line flags.
+  // Populate request context settings for the global system context based on
+  // CefSettings and command-line flags.
   void PopulateRequestContextSettings(CefRequestContextSettings* settings);
 
+  // Verify that |cache_path| is valid and create it if necessary.
+  bool ValidateCachePath(const base::FilePath& cache_path);
+
  private:
   void OnContextInitialized();