Avoid useless call to v8::Isolate::GetCurrent() in ScriptPromiseResolver

Avoid useless call to v8::Isolate::GetCurrent() in ScriptPromiseResolver by
retrieving the isolate from the ScriptExecutionContext.

R=haraken
BUG=263412

Review URL: https://chromiumcodereview.appspot.com/24212004

git-svn-id: svn://svn.chromium.org/blink/trunk@158118 bbb929c8-8fbe-4397-9dbb-9b2b20218538