Just include wxWebViewChromium in the webview library if it's enabled
during the build.
There doesn't seem to be any reason to build a separate library for it,
it's not really different from the other disabled by default wxWebView
backend (Edge).
And now that we don't need to link with an extra library, there is also
no need to have a separate webview_chromium sample, so just add a
possibility to run webview sample itself using Chromium backend by
setting WX_WEBVIEW_BACKEND environment variable.
ae8619b6dd
