Call the correct superclass from RenderFullScreenPlaceholder::willBeDestroyed

[1] implemented RenderFullScreenPlaceholder which inherits from
RenderFlexibleBox but calls RenderBlock::willBeDestroyed() from its
willBeDestroyed function. This is a bug waiting to happen if
RenderFlexibleBox::willBeDestroyed is ever implemented to do something
interesting.

This was discovered while working on crbug.com/415607.


[1] https://src.chromium.org/viewvc/blink?view=rev&revision=166394

TEST=no new tests as there is no change in behavior.

Review URL: https://codereview.chromium.org/585303003

git-svn-id: svn://svn.chromium.org/blink/trunk@182372 bbb929c8-8fbe-4397-9dbb-9b2b20218538