Oilpan: Prepare to move PickerIndicatorElement to Oilpan heap.

BUG=357163

Review URL: https://codereview.chromium.org/273653003

git-svn-id: svn://svn.chromium.org/blink/trunk@173607 bbb929c8-8fbe-4397-9dbb-9b2b20218538