Exclude scripts inlined into sandboxed documents from sanitization.

'window.onerror' currently sanitizes all script errors triggered from
inside a sandboxed document. We should not, however, sanitize the errors
generated from scripts _inlined_ into the sandboxed document.

See step 6 of [1]; the 'muted errors' flag should only be passed into
the script execution algorithm iff the resource was fetched.

[1]: http://www.whatwg.org/specs/web-apps/current-work/multipage/scripting-1.html#execute-the-script-block

BUG=352110

Review URL: https://codereview.chromium.org/196223007

git-svn-id: svn://svn.chromium.org/blink/trunk@169128 bbb929c8-8fbe-4397-9dbb-9b2b20218538