Call WorkerGlobalScope::dispose() before the WorkerGlobalScope object is destructed

(This is a preparation to replace workerGlobalScopeDestroyed() with willBeWorkerGlobalScopeDestroyed().
See https://codereview.chromium.org/238273004/ for more details.)

Currently in the oilpan build, WorkerGlobalScope::dispose() is called before ~WorkerGlobalScope is called.
In the non-oilpan build, WorkerGlobalScope::dispose() is called inside ~WorkerGlobalScope.
This is inconsistent and it is not a good thing to have different call sequences between the oilpan build
and the non-oilpan build.

This CL makes a change so that WorkerGlobalScope::dispose() is called before ~WorkerGlobalScope is called
in the non-oilpan build.

BUG=

Review URL: https://codereview.chromium.org/239313006

git-svn-id: svn://svn.chromium.org/blink/trunk@171739 bbb929c8-8fbe-4397-9dbb-9b2b20218538
rAc: dispatch 'invalid' events when checking validity after autofilling.

R=tkent@chromium.org
BUG=361843

Review URL: https://codereview.chromium.org/233533003

git-svn-id: svn://svn.chromium.org/blink/trunk@171740 bbb929c8-8fbe-4397-9dbb-9b2b20218538