Fix the go-back and proceed actions for client side phishing interstitials.

      These interstitials show after the page has finished loading, so we should treat them similarly to interstitials we show for blocked subresources.  Also, tweak the main InterstitialPage class so that it won't call TabContents::SetIsLoading(true) unless the tab was loading at the time we showed the interstitial.

BUG=none
TEST=tested go back, proceed, and nav-away in the browser

Review URL: http://codereview.chromium.org/7071031

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@87091 0039d316-1c4b-4281-b951-d872f2087c98