Scroll bars continue to scroll when finger leaves the scroll bar.

This was initially committed at
https://codereview.chromium.org/99013004/,
and reverted at
https://codereview.chromium.org/102663003.

The revert was due to some failing content_browsertests. These tests
relied on the previous broken behaviour of the scrollbar. This was fixed
at https://codereview.chromium.org/110403002/, so it should be safe to
reland this patch.

BUG=324479

Review URL: https://codereview.chromium.org/105973004

git-svn-id: svn://svn.chromium.org/blink/trunk@163706 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Split blink::WebSharedWorker interface into two in blink side (phase:1/3)


This is one of cleanup patches.

Goal:
Split blink::WebSharedWorker interface into two interfaces(WebSharedWorker, WebSharedWorkerConnector)
WebSharedWorkerConnector will be used in parent-context side.
WebSharedWorker will be used in worker-context side


Phase 1: in blink (this patch)
- Introduce blink::WebSharedWorkerConnector
- Make blink::WebSharedWorker to be a subclass of blink::WebSharedWorkerConnector
- Move isStarted(), startWorkerContext() and ConnectListener and connect() from blink::WebSharedWorker to blink::WebSharedWorkerConnector.
- Set default value of connect()'s 2nd argument(ConnectListener) to 0.
- Add blink:WebSharedWorkerRepositoryClient::createSharedWorkerConnector() which calls createSharedWorker()

Phase 2: in Chrome
- Change content::WebSharedWorkerProxy to be a subclass of blink::WebSharedWorkerConnector, which is currently a subclass of blink::WebSharedWorker.
- Remove content::WebSharedWorkerProxy::terminateWorkerContext() and content::WebSharedWorkerProxy::clientDestroyed().
- Remove "WebSharedWorker* SharedWorkerRepository::createSharedWorker()"
- Introduce "WebSharedWorkerConnector* SharedWorkerRepository::createSharedWorkerConnector()"

Phase 3: in blink
- Make blink::WebSharedWorker independent from blink::WebSharedWorkerConnector.
- Remove blink::WebSharedWorkerImpl::isStarted()
- Add startWorkerContext() and connect() to blink::WebSharedWorker.
- Remove blink:WebSharedWorkerRepositoryClient::createSharedWorker()
- Remove default value of blink::WebSharedWorkerConnector::connect()'s 2nd argument.


BUG=327600

Review URL: https://codereview.chromium.org/104943005

git-svn-id: svn://svn.chromium.org/blink/trunk@163707 bbb929c8-8fbe-4397-9dbb-9b2b20218538