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/ceftests/navigation_unittest.cc

@@ -1980,7 +1980,7 @@ class LoadNavTestHandler : public TestHandler {
       EXPECT_TRUE(user_gesture);
     }
 
-    EXPECT_EQ(WOD_NEW_BACKGROUND_TAB, target_disposition);
+    EXPECT_EQ(CEF_WOD_NEW_BACKGROUND_TAB, target_disposition);
 
     // OnOpenURLFromTab should be called before OnBeforeBrowse for the file URL.
     EXPECT_FALSE(got_before_browse_);