Refactoring: Move SandboxFlags down to ExecutionContext.

This is a part of effort to shrink down SecurityContext.

BUG=305497
R=dglazkov@chromium.org, abarth, dglazkov, tkent

Review URL: https://codereview.chromium.org/32363002

git-svn-id: svn://svn.chromium.org/blink/trunk@160142 bbb929c8-8fbe-4397-9dbb-9b2b20218538