Ensure DOMWrapperWorld always exists in all webkit_unit_tests

Currently DOMWrapperWorld can be 0 in some tests. This is problematic since I want to make DOMWrapperWorld::current() callable from anywhere. Thus this CL adds DOMWrapperWorld to all webkit_unit_tests.

- It's verbose to initialize v8::HandleScope, v8::Context, v8::Context::Scope, V8PerContextData and DOMWrapperWorld in each test. So this CL adds a helper class (V8BindingTestScope) that does all the initialization work.

- This CL also fixes indentation of V8Binding.h.

BUG=341032

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=168355

R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/180743013

git-svn-id: svn://svn.chromium.org/blink/trunk@168376 bbb929c8-8fbe-4397-9dbb-9b2b20218538