Should call [BKSProcessAssertion invalidate]
https://bugs.webkit.org/show_bug.cgi?id=136804
<rdar://problem/17897780>

Reviewed by Darin Adler.

We should invalidate the process assertion before releasing it. Fortunately this is really easy,
since there is a simple 1:1 relationship with ProcessAssertion, with their lifetimes matching exactly.

* UIProcess/ProcessAssertion.cpp:
(WebKit::ProcessAssertion::~ProcessAssertion):
    - no-op implementation.
* UIProcess/ProcessAssertion.h:
    - added destructor.
* UIProcess/ios/ProcessAssertionIOS.mm:
(WebKit::ProcessAssertion::~ProcessAssertion):
    - destructor invalidates the BKSProcessAssertion.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@173623 268f45cc-cd09-0410-ab3c-d52691b4dbfc