Move FileSystem/FileMetadata.{cpp,h} to Source/platform

Also merges FileSystem.{cpp,h} into FileMetadata.{cpp,h}.

BUG=297477

Review URL: https://codereview.chromium.org/26242002

git-svn-id: svn://svn.chromium.org/blink/trunk@159111 bbb929c8-8fbe-4397-9dbb-9b2b20218538