[chromium] MediaStream API: Make WebMediaStreamDescriptor copyable

[chromium] MediaStream API: Make WebMediaStreamDescriptor copyable
https://bugs.webkit.org/show_bug.cgi?id=76526

Reviewed by Darin Fisher.

This patch adds a copy constructor and a assignment operator to WebMediaStreamDescriptor.

* public/platform/WebMediaStreamDescriptor.h:
(WebKit::WebMediaStreamDescriptor::WebMediaStreamDescriptor):
(WebKit::WebMediaStreamDescriptor::operator=):
* src/WebMediaStreamDescriptor.cpp:
(WebKit::WebMediaStreamDescriptor::assign):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@105318 268f45cc-cd09-0410-ab3c-d52691b4dbfc