ScriptPromiseResolver must be used within a v8 context.

Currently most methods of ScriptPromiseResolver require to be called within a v8 context.
This CL adds comments and assertions to explain the restriction.

R=abarth, haraken
BUG=268276
TESTS=manual

Review URL: https://chromiumcodereview.appspot.com/21878005

git-svn-id: svn://svn.chromium.org/blink/trunk@155600 bbb929c8-8fbe-4397-9dbb-9b2b20218538