Remove isolatedWorldForEnteredContext() and isolatedWorldForIsolate()

- isolatedWorldForIsolate() is equivalent to DOMWrapperWorld::current()->isIsolatedWorld().

- isolatedWorldForEnteredContext() is equivalent to DOMWrapperWorld::current()->isIsolatedWorld(). Remember that the world of the current context is equal to the world of the entered context.

Thus this CL replaces these methods with DOMWrapperWorld::current()->isIsolatedWorld().

BUG=341032
R=dcarney@chromium.org, eseidel@chromium.org

Review URL: https://codereview.chromium.org/137953010

git-svn-id: svn://svn.chromium.org/blink/trunk@167276 bbb929c8-8fbe-4397-9dbb-9b2b20218538