Add accessor for main thread Isolate.
This is required to remove all uses of Isolate::GetCurrent

R=jochen@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/24525002

git-svn-id: svn://svn.chromium.org/blink/trunk@158317 bbb929c8-8fbe-4397-9dbb-9b2b20218538