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/main_context_impl.h

@@ -29,6 +29,7 @@ class MainContextImpl : public MainContext {
   std::string GetMainURL() override;
   cef_color_t GetBackgroundColor() override;
   bool UseChromeRuntime() override;
+  bool UseChromeRuntimeNative() override;
   bool UseViews() override;
   bool UseWindowlessRendering() override;
   bool TouchEventsEnabled() override;
@@ -74,6 +75,7 @@ class MainContextImpl : public MainContext {
   bool use_windowless_rendering_;
   int windowless_frame_rate_ = 0;
   bool use_chrome_runtime_;
+  bool use_chrome_runtime_native_ = false;
   bool use_views_;
 
   std::unique_ptr<RootWindowManager> root_window_manager_;