Use calloc instead of malloc(sizeof * count) in NPV8Object

Using calloc is preferred, since 'sizeof * count' can overflow
resulting in less memory being allocated than expected.
Integer overflow becomes heap buffer overflow. This pattern
is in general readily exploitable.

There was another issue with the code -- it was using
'sizeof(NPIdentifier*)' while in fact allocating an array of
NPIdentifiers. I guess, both types are in fact pointers, but
anyway that wasn't correct.

Review URL: https://codereview.chromium.org/179413005

git-svn-id: svn://svn.chromium.org/blink/trunk@167909 bbb929c8-8fbe-4397-9dbb-9b2b20218538