OwnArrayPtr: Never allow implicit pointer conversion

We should never allow implicit pointer conversion in OwnArrayPtr, as arrays do not interact well with polymorphism.
The following example illustrates possible problem:
OwnArrayPtr<Base> ptr = adoptArrayPtr(new Derived[10]); // sizeof(Derived) > sizeof(Base)
for (int i = 0; i < 10; ++i) {
    ptr[i].func(); // Crashes here.
}
Crash is because sizeof(Base) is used inside OwnArrayPtr::operator[] to access an element by index.

Review URL: https://chromiumcodereview.appspot.com/23849004

git-svn-id: svn://svn.chromium.org/blink/trunk@157770 bbb929c8-8fbe-4397-9dbb-9b2b20218538