Add a shared memory variant of CefProcessMessage (fixes issue #3126)

CefSharedProcessMessageBuilder supports creation of a CefProcessMessage
backed by a CefSharedMemoryRegion.

Performance tests comparing the existing ArgumentList approach and the new
SharedMemoryRegion approach have been added to cefclient at
http://tests/ipc_performance.

CefMessageRouter has been updated to use SharedMemoryRegion as transport
for larger message payloads. The threshold is configurable via
|CefMessageRouterConfig.message_size_threshold|.

To test:
run `ceftests --gtest_filter=SendSharedProcessMessageTest.*:SharedProcessMessageTest.*:MessageRouterTest.Threshold*`

include/wrapper/cef_message_router.h

@@ -207,6 +207,10 @@ struct CefMessageRouterConfig {
   // Name of the JavaScript function that will be added to the 'window' object
   // for canceling a pending query. The default value is "cefQueryCancel".
   CefString js_cancel_function;
+
+  // Messages of size (in bytes) larger than this threshold will be sent via
+  // shared memory region.
+  size_t message_size_threshold;
 };
 
 ///