Make compositing video overlay elements a direct reason

Pretty soon we will use CompositingReasonOverlap and
CompositingReasonAssumedOverlap to indicate whether
we can squash a layer. Video overlay controls
currently use CompositingReasonOverlap, but technically
the reason for compositing them is a direct reason
and they should not be squashed.

BUG=261605

Review URL: https://chromiumcodereview.appspot.com/22117003

git-svn-id: svn://svn.chromium.org/blink/trunk@155710 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Fix clobbered build issue.

TBR=jochen@chromium.org
NOTRY=true
BUG=269698

Review URL: https://chromiumcodereview.appspot.com/22425005

git-svn-id: svn://svn.chromium.org/blink/trunk@155711 bbb929c8-8fbe-4397-9dbb-9b2b20218538