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/chrome/browser_delegate.h

@@ -16,6 +16,7 @@
 #include "ui/base/window_open_disposition.h"
 
 class Browser;
+class Profile;
 
 namespace cef {
 
@@ -40,6 +41,14 @@ class BrowserDelegate : public content::WebContentsDelegate {
 
   ~BrowserDelegate() override {}
 
+  // Optionally override Browser creation in
+  // DevToolsWindow::CreateDevToolsBrowser. The returned Browser, if any, will
+  // take ownership of |devtools_contents|.
+  virtual Browser* CreateDevToolsBrowser(
+      Profile* profile,
+      Browser* opener,
+      std::unique_ptr<content::WebContents>& devtools_contents) = 0;
+
   // Optionally override chrome::AddWebContents behavior. This is most often
   // called via Browser::AddNewContents for new popup browsers and provides an
   // opportunity for CEF to create a new Browser instead of proceeding with