[ChromeDriver] Ignore non-iframe target messages

ChromeDriver is only interested in Target.receivedMessageFromTarget
events from iframe targets. Ignore messages from other types of
targets, instead of returning errors.

Bug: b:76419159
Change-Id: I1e8545aa780d35d2069e4443e674b892ad272151
Reviewed-on: https://chromium-review.googlesource.com/991597Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547609}

chrome/test/chromedriver/chrome/devtools_client_impl.cc

@@ -460,7 +460,10 @@ Status DevToolsClientImpl::ProcessEvent(const internal::InspectorEvent& event) {
           kUnknownError,
           "missing sessionId in Target.receivedMessageFromTarget event");
     if (children_.count(session_id) == 0)
-      return Status(kUnknownError, "unknown sessionId");
+      // ChromeDriver only cares about iframe targets. If we don't know about
+      // this sessionId, then it must be of a different target type and should
+      // be ignored.
+      return Status(kOk);
     DevToolsClientImpl* child = children_[session_id];
     std::string message;
     if (!event.params->GetString("message", &message))