DevTools: Correctly notify open resource handlers about resources that are mapped in workspace.

BUG=289533
R=caseq, pfeldman

Review URL: https://codereview.chromium.org/252703002

git-svn-id: svn://svn.chromium.org/blink/trunk@172728 bbb929c8-8fbe-4397-9dbb-9b2b20218538