Don't call WebContents::DownloadImage() callback if the WebContents were deleted

BUG=583718

Review URL: https://codereview.chromium.org/1685343004

Cr-Commit-Position: refs/heads/master@{#375700}
void WebContentsImpl::DidNavigateMainFramePreCommit(
    bool navigation_is_within_page) {
  if (navigation_is_within_page) {
    return;
  }
  if (IsFullscreenForCurrentTab(GetRenderViewHost()->GetWidget()))
    ExitFullscreen(false);
  DCHECK(!IsFullscreenForCurrentTab(GetRenderViewHost()->GetWidget()));
}

void WebContentsImpl::DidNavigateMainFramePreCommit(
    bool navigation_is_within_page) {
  if (navigation_is_within_page) {
    return;
  }
  if (IsFullscreenForCurrentTab(GetRenderViewHost()->GetWidget()))
    ExitFullscreen(false);
  DCHECK(!IsFullscreenForCurrentTab(GetRenderViewHost()->GetWidget()));
}
