Revert 181702 "Oilpan: Move MediaStreamSource, MediaStreamCompon..."

This CL caused memory leaks in content_unittests.
http://build.chromium.org/p/chromium.memory/builders/Linux%20ASan%20LSan%20Tests%20%282%29/builds/7259

> Oilpan: Move MediaStreamSource, MediaStreamComponent and MediaStreamDescriptor
> to oilpan's heap
> 
> - MediaStreamSource, MediaStreamComponent and MediaStreamDescriptor must be moved
> to oilpan's heap in one go because their lifetime are tightly coupled.
> 
> - Removed m_descriptor->setClient(0) from MediaStream's destructor. This is OK
> because MediaStreamDescriptor has a strong Member back to the Client
> (i.e., MediaStream).
> 
> - Removed m_component->source()->removeObserver(this) from MediaStreamTrack's
> destructor. This is OK because this CL made the observers weak
> (i.e., MediaStreamSource::m_observers is a hash set of weak members to
> MediaStreamTrack objects).
> 
> - Removed WebMediaStreamTrack::ExtraData::m_owner because it's unused.
> 
> - Introduced MediaStreamComponentDisposer to delay the destruction of
> MediaStreamComponent::m_extraData. The ExtraData is exposed to the web
> and some Chromium objects inherit from the ExtraData. If we clear the
> m_extraData in the MediaStreamComponent's destructor, those Chromium-side
> objects are also destructed in the MediaStreamComponent's destructor.
> This is problematic because the destructors of the Chromium-side classes
> can touch other on-heap objects in the Blink side. To avoid the issue,
> we need to delay the destruction of the ExtraData to thread-specific
> weak processing. The disposer pattern actually makes the deletion of
> the extra data happen earlier and not later. The disposer makes sure
> that the extra data is destructed in weak processing which is run before
> sweeping and therefore all the objects are still alive and can be touched.
> 
> 
> - The same problem arises for MediaStreamDescriptor and MediaStreamSource.
> This CL adds MediaStreamDescriptorDisposer and MediaStreamSourceDisposer.
> 
> - This CL depends on https://codereview.chromium.org/543603003/ and https://codereview.chromium.org/549153002.
> 
> BUG=340522
> 
> Review URL: https://codereview.chromium.org/552653005

TBR=haraken@chromium.org

Review URL: https://codereview.chromium.org/562663002

git-svn-id: svn://svn.chromium.org/blink/trunk@181730 bbb929c8-8fbe-4397-9dbb-9b2b20218538