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This was already the case on views and cocoa. However GTK was allowing a drag
after the large icon finishes loading. This CL also updates callers to
use GetTargetFilePath() on the DownloadItem instead of GetFullPath().
For a completed download, both methods are equivalent, but we prefer
the former as it is clear that the path being requested is the final
target path.

BUG=116551


Review URL: https://chromiumcodereview.appspot.com/11801022

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@176159 0039d316-1c4b-4281-b951-d872f2087c98