[ChromeDriver] Avoid deallocating WebView still in use

While dispatching an event to a child frame, place a hold on the WebView
of the child frame to prevent it from getting deallocated.

Bug: chromedriver:3375
Change-Id: I0ff52f6476a2e6767d27c0755a012bdf16dc9c77
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2097538Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Commit-Queue: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749276}

chrome/test/chromedriver/chrome/devtools_client_impl.cc

@@ -498,6 +498,7 @@ Status DevToolsClientImpl::HandleMessage(int expected_id,
     }
     client = it->second;
   }
+  WebViewImplHolder client_holder(client->owner_);
   if (type == internal::kEventMessageType) {
     return client->ProcessEvent(event);
   }

chrome/test/chromedriver/chrome/web_view_impl.cc

@@ -1201,7 +1201,7 @@ std::unique_ptr<base::Value> WebViewImpl::GetCastIssueMessage() {
 }
 
 WebViewImplHolder::WebViewImplHolder(WebViewImpl* web_view)
-    : web_view_(web_view), was_locked_(web_view->Lock()) {}
+    : web_view_(web_view), was_locked_(web_view && web_view->Lock()) {}
 
 WebViewImplHolder::~WebViewImplHolder() {
   if (web_view_ != nullptr && !was_locked_) {