Don't modify Secure Mime Handlers if there's no need.

Don't modify Secure Mime Handlers if there's no need.

When Chrome creates Google Update's OnDemandMachineAppsClass by way of
the elevation moniker, the eventual setup.exe's process security token
doesn't have the SeSecurityPrivilege. This prevents Chrome Frame from
reading and/or modifying the security descriptor on the Secure Mime
Handlers registry key. This had been resulting in error 7 when trying to
manually update a Chrome that shared binaries with Chrome Frame. With
this change, the values are probed before trying to modify them. Chances
are that they will already hold the correct data, so the update should
succeed.

BUG=224043
TEST=see comment #3 in bug

Review URL: https://codereview.chromium.org/25738003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226810 0039d316-1c4b-4281-b951-d872f2087c98