Update scrollable area set when FrameView gets a new scrollable child

When a new scrollable FrameView is added to another FrameView, we should
make sure it is added to the parent's scrollable area set if needed.
Previously the set was only updated if the child FrameView was resized.
This caused a bug where an that iframe is shown, hidden, and shown again
with the same size does not get readded to the scrollable area set.

BUG=388670

Review URL: https://codereview.chromium.org/390373002

git-svn-id: svn://svn.chromium.org/blink/trunk@178189 bbb929c8-8fbe-4397-9dbb-9b2b20218538