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*`

libcef/common/process_message_impl.h

@@ -42,6 +42,9 @@ class CefProcessMessageImpl : public CefProcessMessage {
   CefRefPtr<CefProcessMessage> Copy() override;
   CefString GetName() override;
   CefRefPtr<CefListValue> GetArgumentList() override;
+  CefRefPtr<CefSharedMemoryRegion> GetSharedMemoryRegion() override {
+    return nullptr;
+  }
 
  private:
   const CefString name_;