Some downloaded file types [1] default to opening in a Browser. Open
requests for these file types originating from the Download bubble UI
should route to the source Browser (call OnOpenURLFromTab). If
OnOpenURLFromTab is unhandled proceed with the default Chrome behavior
of opening the URL in a new default Browser.
[1] PDF, html, etc. For the complete list of file types see
ChromeDownloadManagerDelegate::IsOpenInBrowserPreferredForFile.
Set enable_alloy_bootstrap=false to build with Alloy bootstrap code
removed. Extension API is documented as deprecated in comments but
not compiled out with this arg.
