Remove FrameView repaintTree assert.

This ASSERT should become useless once the LayoutState code is fixed. Remove
the ASSERT to make normal development easier.

BUG=320139

Review URL: https://codereview.chromium.org/211033009

git-svn-id: svn://svn.chromium.org/blink/trunk@169971 bbb929c8-8fbe-4397-9dbb-9b2b20218538