add a user-provided constructor to emulated c++11 NullPtr

According to this GCC bug/patch, a user-specified constructor is
required for the emulated C++11 nullptr_t class if we want to be able
to create const nullptr_t objects without an explicit initializer,
a la: "const std::nullptr_t nullptr;"

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=42844

This fixes a build failure in GCC 4.5.

BUG=374431

Review URL: https://codereview.chromium.org/322913002

git-svn-id: svn://svn.chromium.org/blink/trunk@175874 bbb929c8-8fbe-4397-9dbb-9b2b20218538