Fix missing foreground layer invalidations

A foreground layer promotes the content that would have been in the main
graphics layer.  When it gets created or destroyed, the main graphics
layer needs to invalidate the area for the content that gets
promoted/unpromoted from the foreground layer.

BUG=356002

Review URL: https://codereview.chromium.org/218173003

git-svn-id: svn://svn.chromium.org/blink/trunk@170895 bbb929c8-8fbe-4397-9dbb-9b2b20218538