Use v8::Isolate::GetCurrent() less in custom bindings

Update custom bindings to avoids calling v8::Isolate::GetCurrent() when we don't
have to.

R=haraken
BUG=263412

Review URL: https://chromiumcodereview.appspot.com/23606036

git-svn-id: svn://svn.chromium.org/blink/trunk@157962 bbb929c8-8fbe-4397-9dbb-9b2b20218538