(With a v8::Vector, the client is responsible for memory management.
 I think there can be a situation where the Vector has a char[1] backing
 store with '\0' in it, in which case the current code would leak.
 If we always Dispose() the backing store this should be avoided.
 Since dispose will delete[] the actual backing store, this should
 also work then the backing store is nullptr.)

R=jochen@chromium.org
BUG=chromium:525885
LOG=N

Review URL: https://codereview.chromium.org/1410543005

Cr-Commit-Position: refs/heads/master@{#31446}