Treat empty directory properly in skimage.

Instead of treating an empty directory as a file that failed to
decode, treat it as an empty directory.

Add self tests to check for this.

Builds upon from https://codereview.chromium.org/16866010,
which was reverted.

R=epoger@google.com

Review URL: https://codereview.chromium.org/17101029

git-svn-id: http://skia.googlecode.com/svn/trunk@9730 2bbb7eff-a529-9590-31e7-b0007b416f81
6 files changed