Rename isolateFor*() utility functions to toIsolate()

Rename isolateFor*() utility functions to toIsolate() and leverage C++
overloading instead of having separate method names for each kind of
input.

R=abarth
BUG=263412

Review URL: https://chromiumcodereview.appspot.com/23477075

git-svn-id: svn://svn.chromium.org/blink/trunk@157887 bbb929c8-8fbe-4397-9dbb-9b2b20218538