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This will allow debugger to pause on "load", "error" and etc. event breakpoints only
when they happen on XHR event targets.

BUG=381470
R=yurys, pfeldman@chromium.org

Review URL: https://codereview.chromium.org/320933003

git-svn-id: svn://svn.chromium.org/blink/trunk@175882 bbb929c8-8fbe-4397-9dbb-9b2b20218538