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_info_manager.h

@@ -143,6 +143,9 @@ class CefBrowserInfoManager : public content::RenderProcessHostObserver {
                             const content::OpenURLParams& params,
                             CefRefPtr<CefBrowserHostBase>& browser) const;
 
+  static bool ShouldCreateViewsHostedPopup(CefRefPtr<CefBrowserHostBase> opener,
+                                           bool use_default_browser_creation);
+
  private:
   // RenderProcessHostObserver methods:
   void RenderProcessHostDestroyed(content::RenderProcessHost* host) override;