[Drive] Move GetMD5Digest out of chromeos/ directory

 * Move GetMD5Digest out of chromeos/ directory to use from sync_file_system/.
 * Avoid using ReadPlatformFileCurPosNoBestEffort

BUG=240165
NOTRY=true

Review URL: https://codereview.chromium.org/63523005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233593 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233593