Indexed DB: Regression test to ensure Blobs preserve content-type

The Blob type was serialized/restored correctly on the Blink side, but
not on the Chromium side, so when reading a Blob as a stream the
type would be unset.

The Chromium-side fix is: https://codereview.chromium.org/511373002

BUG=408120
R=cmumford@chromium.org

Review URL: https://codereview.chromium.org/509213005

git-svn-id: svn://svn.chromium.org/blink/trunk@181074 bbb929c8-8fbe-4397-9dbb-9b2b20218538