Add InspectorInstrumentation over ExecutionContextTask and async call stacks for DataTransferItem.getAsString.

This instrumentation is needed to support async call stacks in DevTools on various async scenarios.
An example test is for DataTransferItem.getAsString() call.

BUG=272416
R=pfeldman, yurys@chromium.org

Review URL: https://codereview.chromium.org/365253003

git-svn-id: svn://svn.chromium.org/blink/trunk@177772 bbb929c8-8fbe-4397-9dbb-9b2b20218538