Remove IO allowed assert in GetTempDir()

GetTempDir() does not do file IO, therefore it can be used from other threads
to create file paths. No other platform implementation has that assert.

Review URL: https://codereview.chromium.org/76633002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239428 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239428