DevTools: show extension name in console context switcher

The patch does the following:

- Add DOMWrapperWorld.isolatedWorldHumanReadableName, which stores
the name of extension.

- In devtools remote debugging protocol, the executionContext.name
in fact stores execution context origin. The patch sets
"executionContextDecriptor.name" to store the isolated context
human-readable name, and adds "executionContextDescriptor.origin"

BUG=160977

Review URL: https://codereview.chromium.org/15764004

git-svn-id: svn://svn.chromium.org/blink/trunk@181086 bbb929c8-8fbe-4397-9dbb-9b2b20218538