chrome: Support Chrome and DevTools command execution (fixes #3282, fixes #3600)

Add new CefBrowserHost::[Can]ExecuteChromeCommand methods for executing
arbitrary Chrome commands.

Add support for existing CefBrowserHost::ShowDevTools, CloseDevTools and
HasDevTools methods.

DevTools windows now support the same Views callbacks as normal popup
windows with the new CefLifeSpanHandler::OnBeforeDevToolsPopup callback
as the DevTools-specific equivalent of OnBeforePopup.

Always create DevTools as an undocked window to support use of
ShowDevTools with default Chrome browser windows.

To test:

Run `ceftests --enable-chrome-runtime [--use-views]
     --gtest_filter=V8Test.OnUncaughtExceptionDevTools`

OR:

1. Run `cefclient --enable-chrome-runtime [--use-native]`
2. Select "Show DevTools", "Close DevTools" or "Inspect" from the
   right-click menu.
3. Notice that the DevTools window is Views-hosted (or native-hosted)
   and works as expected.

Add --use-default-popup to get a default styled popup in step 3.

libcef/browser/browser_host_base.cc

@@ -505,6 +505,22 @@ void CefBrowserHostBase::PrintToPDF(const CefString& path,
   print_util::PrintToPDF(web_contents, path, settings, callback);
 }
 
+void CefBrowserHostBase::ShowDevTools(const CefWindowInfo& windowInfo,
+                                      CefRefPtr<CefClient> client,
+                                      const CefBrowserSettings& settings,
+                                      const CefPoint& inspect_element_at) {
+  auto params = std::make_unique<CefShowDevToolsParams>(
+      windowInfo, client, settings, inspect_element_at);
+
+  if (!CEF_CURRENTLY_ON_UIT()) {
+    CEF_POST_TASK(CEF_UIT,
+                  base::BindOnce(&CefBrowserHostBase::ShowDevToolsOnUIThread,
+                                 this, std::move(params)));
+  } else {
+    ShowDevToolsOnUIThread(std::move(params));
+  }
+}
+
 bool CefBrowserHostBase::SendDevToolsMessage(const void* message,
                                              size_t message_size) {
   if (!message || message_size == 0) {