Don't update FindBar UI on no-op update

This both saves us the effort of updating the UI, and greatly reduces
the number of live region changed accessibility events sent for the
match count status area.

Bug: 1015818
Change-Id: I1a63d635fcb85f9f401a8fd3f065f4cf49be0d77
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1879433
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709545}

chrome/browser/ui/find_bar/find_notification_details.h

@@ -38,6 +38,14 @@ class FindNotificationDetails {
 
   bool final_update() const { return final_update_; }
 
+  bool operator==(const FindNotificationDetails& other) const {
+    return std::tie(request_id_, number_of_matches_, selection_rect_,
+                    active_match_ordinal_, final_update_) ==
+           std::tie(other.request_id_, other.number_of_matches_,
+                    other.selection_rect_, other.active_match_ordinal_,
+                    other.final_update_);
+  }
+
  private:
   int request_id_;    // The find-in-page request whose results we're returning.
   int number_of_matches_;  // How many matches were found.

chrome/browser/ui/views/find_bar_view.cc

@@ -88,6 +88,9 @@ class FindBarView::MatchCountLabel : public views::Label {
   }
 
   void SetResult(const FindNotificationDetails& result) {
+    if (last_result_ && result == *last_result_)
+      return;
+
     last_result_ = result;
     SetText(l10n_util::GetStringFUTF16(
         IDS_FIND_IN_PAGE_COUNT,