Defer "scriptParsed" until "Page.getResourceTree"

DevTools front-end expects to receive Page.getResourceTree response
before any "scriptParsed" events. When "scriptParsed" arrives first,
the source-map handler throws an exception.

Add FrontendClient methods pauseEvents() and resumeEvents().

Change PageAgent to EventEmitter, add a "resource-tree" event that
is emitted at the time when the Page.getResourceTree response is sent.

FrontendCommandHandler uses these two features to pause events on start
and resume them on "resource-tree".