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.

tests/cefclient/browser/client_handler.h

@@ -224,6 +224,12 @@ class ClientHandler : public CefClient,
       CefBrowserSettings& settings,
       CefRefPtr<CefDictionaryValue>& extra_info,
       bool* no_javascript_access) override;
+  void OnBeforeDevToolsPopup(CefRefPtr<CefBrowser> browser,
+                             CefWindowInfo& windowInfo,
+                             CefRefPtr<CefClient>& client,
+                             CefBrowserSettings& settings,
+                             CefRefPtr<CefDictionaryValue>& extra_info,
+                             bool* use_default_window) override;
   void OnAfterCreated(CefRefPtr<CefBrowser> browser) override;
   bool DoClose(CefRefPtr<CefBrowser> browser) override;
   void OnBeforeClose(CefRefPtr<CefBrowser> browser) override;
@@ -348,9 +354,8 @@ class ClientHandler : public CefClient,
   friend class ClientDownloadImageCallback;
 
   // Create a new popup window using the specified information. |is_devtools|
-  // will be true if the window will be used for DevTools. Return true to
-  // proceed with popup browser creation or false to cancel the popup browser.
-  // May be called on any thead.
+  // will be true if the window will be used for DevTools. Returns true if a
+  // RootWindow was created for the popup.
   bool CreatePopupWindow(CefRefPtr<CefBrowser> browser,
                          bool is_devtools,
                          const CefPopupFeatures& popupFeatures,