DevTools: Support ES6 generator objects in DevTools UI.

This implements:
- showing current location of suspended generator object in popover
- context menu "Show Generator Location" to jump to the last "yield" or to generator function definition

Drive-by: Added security safety check in InjectedScript so that we do not mix objects from different world.
We do this check on C++ level also, so this is another layer of protection.

BUG=437221
R=loislo, vsevik, yurys

Review URL: https://codereview.chromium.org/761123003

git-svn-id: svn://svn.chromium.org/blink/trunk@186412 bbb929c8-8fbe-4397-9dbb-9b2b20218538
DevTools: Prepare some ElementsTreeElement methods to be moved to ElementsTreeUpdater

Added node or treeElement as a parameter to these methods so that we could move them to the updater (almost) as this.
Also extracted a few new methods to simplify the move.

R=pfeldman

Review URL: https://codereview.chromium.org/769063002

git-svn-id: svn://svn.chromium.org/blink/trunk@186413 bbb929c8-8fbe-4397-9dbb-9b2b20218538