[DevTools] Do not allow Page.setDownloadBehavior for extensions

Bug: 866426
Change-Id: I71b672978e1a8ec779ede49da16b21198567d3a4
Reviewed-on: https://chromium-review.googlesource.com/c/1270007
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#598004}
void PageHandler::RenderWidgetHostVisibilityChanged(
    RenderWidgetHost* widget_host,
    bool became_visible) {
  if (!screencast_enabled_)
    return;
  NotifyScreencastVisibility(became_visible);
}

void PageHandler::RenderWidgetHostVisibilityChanged(
    RenderWidgetHost* widget_host,
    bool became_visible) {
  if (!screencast_enabled_)
    return;
  NotifyScreencastVisibility(became_visible);
}
