Remove BlobRegistry indirection since there is only one implementation.

BUG=

Review URL: https://chromiumcodereview.appspot.com/15851008

git-svn-id: svn://svn.chromium.org/blink/trunk@152746 bbb929c8-8fbe-4397-9dbb-9b2b20218538
BlobRegistry& blobRegistry()
{
    ASSERT(isMainThread());
    DEFINE_STATIC_LOCAL(BlobRegistryProxy, instance, ());
    return instance;
}

BlobRegistry& blobRegistry()
{
    ASSERT(isMainThread());
    DEFINE_STATIC_LOCAL(BlobRegistryProxy, instance, ());
    return instance;
}
