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/browser_info.cc

@@ -33,6 +33,16 @@ CefBrowserInfo::CefBrowserInfo(int browser_id,
       is_windowless_(is_windowless),
       extra_info_(extra_info) {
   DCHECK_GT(browser_id, 0);
+
+  if (extra_info_ && !extra_info_->IsReadOnly()) {
+    // |extra_info_| should always be read-only to avoid accidental future
+    // modification. Take a copy instead of modifying the passed-in object for
+    // backwards compatibility.
+    extra_info_ = extra_info_->Copy(/*exclude_empty_children=*/false);
+    auto extra_info_impl =
+        static_cast<CefDictionaryValueImpl*>(extra_info_.get());
+    extra_info_impl->MarkReadOnly();
+  }
 }
 
 CefBrowserInfo::~CefBrowserInfo() {