Move WriteFile and WriteFileDescriptor from file_util to base namespace.

R=viettrungluu@chromium.org
TBR=viettrungluu

Review URL: https://codereview.chromium.org/184563006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255418 0039d316-1c4b-4281-b951-d872f2087c98