Use TClass::GetClass(), not gROOT->GetClass()

This is a trivial technical change. It is being requested for standardization, and for code commonality with the ROOT6 7_3_X branch, as these changes have already been made there. gROOT->GetClass(...) is obsolete. It just calls TClass::GetClass(...). Instead, calling TClass::GetClass() directly is recommended by the ROOT team, and it saves a function call.

该提问来源于开源项目：cms-sw/cmssw