Fix C++ type of Factory::NewFixedDoubleArray.

The change fixes the C++ type of Factory::NewFixedDoubleArray to
reflect the empty array case, where we return an empty
FixedArray (rather than FixedDoubleArray).

R=mvstanton@chromium.org
BUG=

Review URL: https://codereview.chromium.org/249593002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20918 ce2b1a6d-e550-0410-aec6-3dcde31c8c00