<chromium> Fix building with MacOSX10.10.sdk.

The compiler shipping with newer SDKs is more restrictive
with regards to type conversion. It is not accepted
anymore to pass nil (nullptr) for NSUInteger or BOOL.

Change-Id: I15fbfe480aab42e505cec0c6fa47feed718f31c1
Reviewed-by: Michael Bruning <michael.bruning@digia.com>
Reviewed-by: Andras Becsi <andras.becsi@digia.com>