[Mac] fixup select elements.

We use RenderWidgetHostViewQt, not RWHVMac, even on Mac.
Since some platform decisions seem to have transpired all the way to
the content layer, we should simply use the web ui for select elements
on mac for the time being.

Change-Id: Iefe1dd3b246fc210ac18ff9eb0e7ad498c061eac
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>
[Backport] Use compositor for page popups.

This is essentially the last user of the legacy 2d path.

BUG=271140

Review URL: https://codereview.chromium.org/171343003

Change-Id: I64911d0d83b9d1d4782dff9b09db1bbcc439da24
git-svn-id: svn://svn.chromium.org/blink/trunk@167552 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>