close tab if the download is the initial navigation

[bridiver]

fix #9620

Still needs a test
also opened brave/muon#225 to move this to muon

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.

Test Plan:
See issue

Reviewer Checklist:

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[bridiver]

from chrome download_ui

content::WebContents* web_contents = item->GetWebContents();
  if (web_contents) {
    Browser* browser = chrome::FindBrowserWithWebContents(web_contents);
    // If the download occurs in a new tab, and it's not a save page
    // download (started before initial navigation completed) close it.
    // Avoid calling CloseContents if the tab is not in this browser's tab strip
    // model; this can happen if the download was initiated by something
    // internal to Chrome, such as by the app list.
    if (browser && web_contents->GetController().IsInitialNavigation() &&
        browser->tab_strip_model()->count() > 1 &&
        browser->tab_strip_model()->GetIndexOfWebContents(web_contents) !=
            TabStripModel::kNoTab &&
        !item->IsSavePackageDownload()) {
      web_contents->Close();
    }