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/net_service/browser_urlrequest_impl.cc

@@ -202,14 +202,8 @@ class CefBrowserURLRequest::Context
       scoped_refptr<base::SequencedTaskRunner> task_runner) {
     CEF_REQUIRE_UIT();
 
-    // Get or create the request context and browser context.
-    CefRefPtr<CefRequestContextImpl> request_context_impl =
-        CefRequestContextImpl::GetOrCreateForRequestContext(request_context);
-    CHECK(request_context_impl);
-    CefBrowserContext* cef_browser_context =
-        request_context_impl->GetBrowserContext();
-    CHECK(cef_browser_context);
-    auto browser_context = cef_browser_context->AsBrowserContext();
+    auto* browser_context =
+        CefRequestContextImpl::GetBrowserContext(request_context);
     CHECK(browser_context);
 
     scoped_refptr<net_service::URLLoaderFactoryGetter> loader_factory_getter;