Always set scroll layer content size after compositing change update.

The bounds must be set in here for pinch virtual viewport as well as the
old path since this is the path through which the layer's bounds are
updated while the page is loading.

BUG=423188

Review URL: https://codereview.chromium.org/666083002

git-svn-id: svn://svn.chromium.org/blink/trunk@184117 bbb929c8-8fbe-4397-9dbb-9b2b20218538