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/ceftests/v8_unittest.cc

@@ -11,10 +11,8 @@
 #include "tests/ceftests/test_handler.h"
 #include "tests/ceftests/test_util.h"
 #include "tests/gtest/include/gtest/gtest.h"
-#include "tests/shared/browser/client_app_browser.h"
 #include "tests/shared/renderer/client_app_renderer.h"
 
-using client::ClientAppBrowser;
 using client::ClientAppRenderer;
 
 // How to add a new test:
@@ -3280,8 +3278,7 @@ class V8TestHandler : public TestHandler {
         CefBrowserSettings settings;
 
 #if defined(OS_WIN)
-        windowInfo.SetAsPopup(browser->GetHost()->GetWindowHandle(),
-                              "DevTools");
+        windowInfo.SetAsPopup(nullptr, "DevTools");
 #endif
 
         browser->GetHost()->ShowDevTools(windowInfo, this, settings,