Early return the Clipboard Url Provider when the omnibox wasn't focused.

This mirrors what other similar providers do:
https://code.google.com/p/chromium/codesearch#chromium/src/chrome/browser/autocomplete/zero_suggest_provider.cc&rcl=1428507553&l=98

BUG=None.

Review URL: https://codereview.chromium.org/1072003002

Cr-Commit-Position: refs/heads/master@{#324427}

components/open_from_clipboard/clipboard_url_provider.cc

@@ -31,6 +31,8 @@ void ClipboardURLProvider::Start(const AutocompleteInput& input,
                                  bool minimal_changes,
                                  bool called_due_to_focus) {
   matches_.clear();
+  if (!called_due_to_focus)
+    return;
   // Attempt to add an AutocompleteMatch only if the user has not entered
   // anything in the omnibox.
   if (input.cursor_position() == base::string16::npos) {