Don't propagate sandbox flags to an opened window unless triggered entirely by script.

This was regressed in http://src.chromium.org/viewvc/blink?revision=161843&view=revision,
when we started propagating sandbox flags to new windows via FrameLoader::forceSandboxFlags
even if the new window was caused by clicking a link with target="_blank".

BUG=353253
TEST=http/tests/navigation/new-window-sandboxed-iframe.html

Review URL: https://codereview.chromium.org/208853004

git-svn-id: svn://svn.chromium.org/blink/trunk@169970 bbb929c8-8fbe-4397-9dbb-9b2b20218538