FileAPI Blink: Enable FileSystemURL-backed Blobs by removing an ASSERT.

I'm doing this to support streaming MTP blobs. This should also allow remote Blobs without snapshot files in general though.

Implementation plan: https://docs.google.com/a/chromium.org/document/d/16YJVUVFRc8z-q68KwoXP2s9aUIDIOSNfNPuGk83zotQ/edit?usp=sharing

BUG=318450,110119

Review URL: https://codereview.chromium.org/106893002

git-svn-id: svn://svn.chromium.org/blink/trunk@163303 bbb929c8-8fbe-4397-9dbb-9b2b20218538