[Breadcrumbs] Improve WebStateActivatedAt logging

Use the following format:
Switch [from Tab<id>] [to Tab<id> at 0]

Old example:
Activated Tab1 for user action Tab-1 at -1

New example of the same log:
Switch to Tab1 at 0 #user-action

Do not log events unless WebState activation was
triggered by tab switch (the rest of activation reasons
are covered by other WebStateListObserver callbacks).

Bug: 1046231
Change-Id: I1c8657de6813bc62a719fed7add55211f9e6ffeb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2084151Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746919}

ios/chrome/browser/crash_report/breadcrumbs/breadcrumb_manager_browser_agent.mm

@@ -114,34 +114,24 @@ void BreadcrumbManagerBrowserAgent::WebStateActivatedAt(
     web::WebState* new_web_state,
     int active_index,
     int reason) {
-  int old_web_state_id = -1;
+  if (!(reason & WebStateListObserver::CHANGE_REASON_ACTIVATED)) {
+    return;
+  }
+
+  std::vector<std::string> event = {"Switch"};
   if (old_web_state) {
-    old_web_state_id =
-        BreadcrumbManagerTabHelper::FromWebState(old_web_state)->GetUniqueId();
+    event.push_back(base::StringPrintf(
+        "from Tab%d", BreadcrumbManagerTabHelper::FromWebState(old_web_state)
+                          ->GetUniqueId()));
   }
-  int new_web_state_id = -1;
   if (new_web_state) {
-    new_web_state_id =
-        BreadcrumbManagerTabHelper::FromWebState(new_web_state)->GetUniqueId();
+    event.push_back(base::StringPrintf(
+        "to Tab%d at %d",
+        BreadcrumbManagerTabHelper::FromWebState(new_web_state)->GetUniqueId(),
+        active_index));
   }
 
-  const char* change_reason_string = nullptr;
-  switch (reason) {
-    case WebStateListObserver::ChangeReason::CHANGE_REASON_NONE:
-      change_reason_string = "with";
-      break;
-    case WebStateListObserver::ChangeReason::CHANGE_REASON_REPLACED:
-      change_reason_string = "replaced";
-      break;
-    case WebStateListObserver::ChangeReason::CHANGE_REASON_ACTIVATED:
-    case WebStateListObserver::ChangeReason::CHANGE_REASON_CLOSED:
-    case WebStateListObserver::ChangeReason::CHANGE_REASON_INSERTED:
-      change_reason_string = "for user action";
-      break;
-  }
-  LogEvent(base::StringPrintf("Activated Tab%d %s Tab%d at %d",
-                              old_web_state_id, change_reason_string,
-                              new_web_state_id, active_index));
+  LogEvent(base::JoinString(event, " "));
 }
 
 void BreadcrumbManagerBrowserAgent::WillBeginBatchOperation(