Cast std::chrono::duration.count() to int64_t in ArgumentCoder
https://bugs.webkit.org/show_bug.cgi?id=136981

Patch by Ting-Wei Lan <lantw44@gmail.com> on 2014-10-28
Reviewed by Alexey Proskuryakov.

Explicitly cast the return value of std::chrono::duration.count() to
a fixed-size interger type, which prevents compilation error when
the return value type matches neither int32_t nor int64_t.

* Platform/IPC/ArgumentCoders.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@175299 268f45cc-cd09-0410-ab3c-d52691b4dbfc