Mac: turn popups into new tabs while in fullscreen.

It's platform convention to show popups as new tabs while in
non-HTML5 fullscreen. (Popups cause tabs to lose HTML5 fullscreen.)

This was implemented for Cocoa in a BrowserWindow override, but
it makes sense to just stick it into Browser and remove a ton
of override code put in just to support this.

BUG=858929, 868416
TEST=as in bugs

Change-Id: I43471f242813ec1159d9c690bab73dab3e610b7d
Reviewed-on: https://chromium-review.googlesource.com/1153455
Reviewed-by: Sidney San Martín <sdy@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578755}
ui::WindowShowState TestBrowserWindow::GetRestoredState() const {
  return ui::SHOW_STATE_DEFAULT;
}

ui::WindowShowState TestBrowserWindow::GetRestoredState() const {
  return ui::SHOW_STATE_DEFAULT;
}
