V8TestingScope should use the main world instead of a newly created world

V8TestingScope is a mock class for binding unit tests.
V8TestingScope creates a new world for a test, but it causes an issue
if the test tries to access the world via a ScriptController
because the world is not yet registered to the ScriptController.
(See https://codereview.chromium.org/535583002/#msg19 for more details.)

This CL fixes the issue by making V8TestingScope use the main world
(which is guaranteed to be registered to the ScriptController)
instead of a newly created world.

Review URL: https://codereview.chromium.org/534233003

git-svn-id: svn://svn.chromium.org/blink/trunk@181379 bbb929c8-8fbe-4397-9dbb-9b2b20218538
[blink] Remove XMLViewer.js/css post blink-in-js migration of XML tree viewer.

CL, https://codereview.chromium.org/479753003, migrated the usage of XML tree viewer to
be implemented in blink-in-js. Post the migration, the files XMLViewer.js/css are no longer
used by any native code directly and hence should be removed.

TBR=jochen@chromium.org

Review URL: https://codereview.chromium.org/539653002

git-svn-id: svn://svn.chromium.org/blink/trunk@181380 bbb929c8-8fbe-4397-9dbb-9b2b20218538