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.

include/cef_browser.h

@@ -988,6 +988,24 @@ class CefBrowserHost : public virtual CefBaseRefCounted {
   ///
   /*--cef()--*/
   virtual void ExitFullscreen(bool will_cause_resize) = 0;
+
+  ///
+  /// Returns true if a Chrome command is supported and enabled. Values for
+  /// |command_id| can be found in the cef_command_ids.h file. This method can
+  /// only be called on the UI thread. Only used with the Chrome runtime.
+  ///
+  /*--cef()--*/
+  virtual bool CanExecuteChromeCommand(int command_id) = 0;
+
+  ///
+  /// Execute a Chrome command. Values for |command_id| can be found in the
+  /// cef_command_ids.h file. |disposition| provides information about the
+  /// intended command target. Only used with the Chrome runtime.
+  ///
+  /*--cef()--*/
+  virtual void ExecuteChromeCommand(
+      int command_id,
+      cef_window_open_disposition_t disposition) = 0;
 };
 
 #endif  // CEF_INCLUDE_CEF_BROWSER_H_