Introduce a new reference graph to SVG*TearOffs.

This patch introduces the new reference graph for SVG*TearOffs to resolve Node leaks. The graph is illustrated below:
https://docs.google.com/presentation/d/1cKqC9wc_Ca5UTSCNE0DHQeXRxEtkerIxnp1MIIK5Jqk/edit?usp=sharing
In the new reference graph, references from parent to child are made strong ref, and references from child to parent are made weak ref. For V8 references against TearOffs, a hidden reference to the context V8SVGElement is added to its wrappers to ensure contextElement() is alive while TearOffs are alive.

This patch also contains fixes for bugs exposed from the reference graph change. SVGAnimateElement will ensure stopAnimValAnimation call even if it is destructed while animation is active. SMILElement/SVGAnimateElement is modified to observe lifetime shadow tree instances in addition to the lifetime of original element.

BUG=270000

Review URL: https://chromiumcodereview.appspot.com/23521003

git-svn-id: svn://svn.chromium.org/blink/trunk@157949 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Pass isolate to ScriptPromise and ScriptString constructors

Pass isolate to ScriptPromise and ScriptString constructors to avoid calls
to v8::Isolate::GetCurrent(). Also add an m_isolate member to ScriptValue
to avoid avoid calls to v8::Isolate::GetCurrent() and simplify the API.

R=haraken
BUG=263412

Review URL: https://chromiumcodereview.appspot.com/23513066

git-svn-id: svn://svn.chromium.org/blink/trunk@157950 bbb929c8-8fbe-4397-9dbb-9b2b20218538
[DevTools] Resources panel SideBarTreeElement should show expand/collapse option only if it has children

Currently Resources panel sidebar tree elements show expand/collapse option irrespective of
them having children or not. This could be misunderstood as an error in inspector.

This change fixes the same by providing expand/collapse option only when the tree elements
have children.

R=pfeldman

Review URL: https://chromiumcodereview.appspot.com/24109003

git-svn-id: svn://svn.chromium.org/blink/trunk@157951 bbb929c8-8fbe-4397-9dbb-9b2b20218538