WebViewChromium: don't use a separate class loader.

Use the ordinary system class loader to load the Chromium-based WebView
implementation classes now that the implementation .jar is part of the
boot class path. Instead of checking for the .jar file's existence, just
check to see if we can load the class to determine whether the new
WebView is available.

This also removes the check for Build.IS_DEBUGGABLE; the Chromium-based
WebView will now be available in user builds if enabled in the developer
options.

Change-Id: I06bae9b91656aa41b09c0e6f63f90736db993418