[WebBlobRegistry] Use char* than WebThreadSafeData

We don't need to wrap the data with WebThreadSafeData here. Just use a
pair of const char* and size_t.

This is Blink side CL.

Chromium side CL: https://codereview.chromium.org/610333002/

R=yhirano,tkent
BUG=417093

Review URL: https://codereview.chromium.org/611923002

git-svn-id: svn://svn.chromium.org/blink/trunk@184141 bbb929c8-8fbe-4397-9dbb-9b2b20218538