[WTF] Introduce WTF::RandomDevice which keeps /dev/urandom opened

[WTF] Introduce WTF::RandomDevice which keeps /dev/urandom opened
https://bugs.webkit.org/show_bug.cgi?id=170095

Reviewed by Michael Catanzaro.

In this patch, we introduce RandomDevice, which keeps /dev/urandom opened
to avoid repeatedly open and close urandom file descriptor in Linux.

The purpose is similar to std::random_device, but WTF::RandomDevice explicitly
avoids using ARC4, which is recently attempted to be removed from the WebKit
tree[1].

[1]: https://trac.webkit.org/r214329

* WTF.xcodeproj/project.pbxproj:
* wtf/CMakeLists.txt:
* wtf/OSRandomSource.cpp:
(WTF::cryptographicallyRandomValuesFromOS):
(WTF::crashUnableToOpenURandom): Deleted.
(WTF::crashUnableToReadFromURandom): Deleted.
* wtf/RandomDevice.cpp: Copied from Source/WTF/wtf/OSRandomSource.cpp.
(WTF::crashUnableToOpenURandom):
(WTF::crashUnableToReadFromURandom):
(WTF::RandomDevice::RandomDevice):
(WTF::RandomDevice::~RandomDevice):
(WTF::RandomDevice::cryptographicallyRandomValues):
* wtf/RandomDevice.h: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@214867 268f45cc-cd09-0410-ab3c-d52691b4dbfc