[ChromeDriver] Fix crash when target isn't tracked

While handling Target.detachedFromTarget events, check that the
target_id exists in frame_to_target_map_ before using the target.

Change-Id: I84d41913bdc2df02338f725eef2b0a513c0c5d4d
Reviewed-on: https://chromium-review.googlesource.com/1013613
Commit-Queue: John Chen <johnchen@chromium.org>
Reviewed-by: Jonathon Kereliuk <kereliuk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551065}

chrome/test/chromedriver/chrome/frame_tracker.cc

@@ -178,8 +178,12 @@ Status FrameTracker::OnEvent(DevToolsClient* client,
       // Some types of Target.detachedFromTarget events do not have targetId.
       // We are not interested in those types of targets.
       return Status(kOk);
-    WebViewImpl* target =
-        static_cast<WebViewImpl*>(frame_to_target_map_[target_id].get());
+    auto target_iter = frame_to_target_map_.find(target_id);
+    if (target_iter == frame_to_target_map_.end())
+      // There are some target types that we're not keeping track of, thus not
+      // finding the target in frame_to_target_map_ is OK.
+      return Status(kOk);
+    WebViewImpl* target = static_cast<WebViewImpl*>(target_iter->second.get());
     if (target->IsLocked())
       target->SetDetached();
     else