Object, COFF: Move the VirtualSize/SizeOfRawData logic to getSectionSize

While getSectionContents was updated to do the right thing,
getSectionSize wasn't.  Move the logic to getSectionSize and leverage it
from getSectionContents.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219391 91177308-0d34-0410-b5e6-96231b3b80d8