Run GC in Resource-related inspector tests.

In Oilpan, orphan Resource objects are not destructed immediately after
internals.garbageCollectDocumentResources(document) or
NetworkAgent.setCacheDisabled(true). We need to run GC to pass tests.

BUG=398466

Review URL: https://codereview.chromium.org/430413002

git-svn-id: svn://svn.chromium.org/blink/trunk@179462 bbb929c8-8fbe-4397-9dbb-9b2b20218538