Fix file permissions in save_as mode on POSIX.
Fix file permissions in save_as mode on POSIX.

BUG=36107
TEST=Check 'Ask where to save each file before downloading' in 'Under the Hood' preference, start a download and check the file permission.

Review URL: http://codereview.chromium.org/672019

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@40977 0039d316-1c4b-4281-b951-d872f2087c98

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