If JavaScript shows a dialog, cause the page to lose fullscreen.

BUG=670135, 550017, 726761, 728276

Review-Url: https://codereview.chromium.org/2906133004
Cr-Commit-Position: refs/heads/master@{#478884}
void WebContentsImpl::RemoveDestructionObserver(WebContentsImpl* web_contents) {
  destruction_observers_.erase(web_contents);
}

void WebContentsImpl::RemoveDestructionObserver(WebContentsImpl* web_contents) {
  destruction_observers_.erase(web_contents);
}
