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/common/value_base.h

@@ -296,6 +296,13 @@ class CefValueBase : public CefType, public CefValueController::Object {
   // True if access to the underlying value is read-only.
   inline bool read_only() const { return read_only_; }
 
+  // Convert a writable value to read-only. The reverse could be surprising and
+  // is therefore not supported.
+  void MarkReadOnly() {
+    DCHECK(!read_only_);
+    read_only_ = true;
+  }
+
   // True if the underlying value has been detached.
   inline bool detached() const { return !controller_.get(); }