Only Google Chrome should be able to send feedback via the Feedback UI. On Chromium builds, the UI cannot be invoked but is still compiled, and the extension is still included.

We should wrap the extension resources and any Feedback UI with include guards for #defined(GOOGLE_CHROME_BUILD).

Some other parts of the Feedback component are always used, e.g. by private extensions, but we should at least be able to exclude:

* Web:
  chrome/browser/ui/webui/md_feedback/ (new MD UI)
  chrome/browser/resources/feedback (extension)

* C++:
  chrome/browser/feedback/{show_feedback_page.cc,feedback_dialog_utils.*}
  chrome/browser/extensions/api/feedback_private (the parts specific to the component extension)
  components/feedback/ (we may need to keep a few header files for their types,
      e.g. for use in chrome://system, but since those aren't Feedback-specific
      maybe we can relocate them

* Strings:
  All feedback-related strings in chrome/app/generated_resources.grd