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.

tests/cefclient/browser/main_context_impl.cc

@@ -111,8 +111,9 @@ MainContextImpl::MainContextImpl(CefRefPtr<CefCommandLine> command_line,
   }
 
 #if defined(OS_WIN) || defined(OS_LINUX)
-  if (use_chrome_runtime_ && !use_views_ &&
-      !command_line->HasSwitch(switches::kUseNative)) {
+  use_chrome_runtime_native_ =
+      use_chrome_runtime_ && command_line->HasSwitch(switches::kUseNative);
+  if (use_chrome_runtime_ && !use_views_ && !use_chrome_runtime_native_) {
     LOG(WARNING) << "Chrome runtime defaults to the Views framework.";
     use_views_ = true;
   }
@@ -174,6 +175,10 @@ bool MainContextImpl::UseChromeRuntime() {
   return use_chrome_runtime_;
 }
 
+bool MainContextImpl::UseChromeRuntimeNative() {
+  return use_chrome_runtime_native_;
+}
+
 bool MainContextImpl::UseViews() {
   return use_views_;
 }