Add google_update::GetUntrustedDataValueFromTag()

This is similar to GetUntrustedDataValue() but allows
extracting a value from an arbitrary tag.

BUG=341353

Review URL: https://codereview.chromium.org/180243021

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256091 0039d316-1c4b-4281-b951-d872f2087c98
Removes --ash-disable-drag-off-shelf flag

BUG=284772

Review URL: https://codereview.chromium.org/191413002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256092 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256092