Do not report Resource Timing for iframe navigations when restored from history

      In a back/forward navigation, iframe navigates to its final URL directly.
Iframes should not report Resource Timing except for the initial
navigation requested by the parent document.

BUG=511616
TEST=http/tests/misc/resource-timing-iframe-restored-from-history.html

Review URL: https://codereview.chromium.org/1257953003

git-svn-id: svn://svn.chromium.org/blink/trunk@199553 bbb929c8-8fbe-4397-9dbb-9b2b20218538