Blink Blobs: Allow creation of WebBlob from just UUID.

This is to support Blobs created from the browser process. This is in support of patch https://codereview.chromium.org/250143002/. See the delta from Patchset 5 to Patchset 6.

BUG=304290

Review URL: https://codereview.chromium.org/259423002

git-svn-id: svn://svn.chromium.org/blink/trunk@173211 bbb929c8-8fbe-4397-9dbb-9b2b20218538