Introduce WebInspector.HeapSnapshotView.Perspective

Decendants of the class are used to show certain UI views corrsponding to a heap snapshot view. We have several views (retainment, containment, constructors etc.) some of which may be hidden depnding on the current perspective.

BUG=None
R=alph@chromium.org

Review URL: https://codereview.chromium.org/196723012

git-svn-id: svn://svn.chromium.org/blink/trunk@169137 bbb929c8-8fbe-4397-9dbb-9b2b20218538