Remove a failing ASSERT(paintInvalidationContainer)

Remove a failing ASSERT(paintInvalidationContainer)

This assert is the cause of a flakiness in a fullscreen test:
http://test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=virtual%2Fandroid%2Ffullscreen%2Fmodel%2Ffully-exit-fullscreen-nested-iframe.html

paintInvalidationContainer can be null here, when coming from
RenderObject::selectionRect().

Also note that immediately preceding code checks
!paintInvalidationContainer, which wouldn't make sense if it were
guaranteed to not be null.

Code review which lead to the inclusion of the assert:
https://codereview.chromium.org/445793002/diff/80001/Source/core/rendering/RenderView.cpp#newcode512

BUG=408495
TEST=LayoutTests/virtual/android/fullscreen/model/fully-exit-fullscreen-nested-iframe.html
R=chrishtr@chromium.org,leviw@chromium.org

[philipj_slow, 2014-08-28 14:15:49]

PTAL. If there's a quick way to get rid of the null case instead that's great,
this is my quickfix with limited domain knowledge.