Add plugin placeholder and policy support (issue #1708)

- Default plugin loading policy can be specified using the new
  `--plugin-policy=[allow|block|detect]` command-line flag.
- Move CefRequestHandler::OnBeforePluginLoad to
  CefRequestContextHandler and add a new policy argument that
  supports different actions (allow, block, detect, disable) on a
  per-plugin-instance basis.
- Add CefContextMenuHandler::RunContextMenu for providing a custom
  context menu implementation.
- Add CefResourceBundleHandler::GetDataResourceForScale for
  returning scaled resources (issue #1272).
- Add CefResourceBundle for retrieving resources from the resource
  bundle (*.pak) files loaded by CEF during startup or via the
  CefResourceBundleHandler.
- Linux: Fix Debug build IO access warning with CefGetMimeType.
- cef_unittests: Move the refcounting implementation from TestHandler
  to subclasses in order to support interface inheritance from
  subclasses.

libcef/common/parser_impl.cc

@@ -8,6 +8,7 @@
 #include "libcef/renderer/webkit_glue.h"
 
 #include "base/base64.h"
+#include "base/threading/thread_restrictions.h"
 #include "net/base/escape.h"
 #include "net/base/mime_util.h"
 #include "third_party/WebKit/public/platform/WebString.h"
@@ -76,6 +77,10 @@ bool CefCreateURL(const CefURLParts& parts,
 }
 
 CefString CefGetMimeType(const CefString& extension) {
+  // Requests should not block on the disk!  On POSIX this goes to disk.
+  // http://code.google.com/p/chromium/issues/detail?id=59849
+  base::ThreadRestrictions::ScopedAllowIO allow_io;
+
   std::string mime_type;
   net::GetMimeTypeFromExtension(extension, &mime_type);
   return mime_type;