Don't try to verify XML well-formedness for partial SVG uploads

Don't try to verify XML well-formedness for partial SVG uploads

Chunked uploads of SVGs are currently failing with "invalid XML" errors
because UploadBase::detectScriptInSvg() requires the full file but is
being called from UploadBase::verifyPartialFile().

So let's do the check twice: once in UploadBase::verifyPartialFile()
where it will pass if non-well-formed, and once in
UploadBase::verifyFile() where it will fail if non-well-formed.

Bug: 65724
Change-Id: I6126e185eb4b183c31946f13c576521f1ed19c16