Remove GetActiveEntry usage from content.

      This is a copy of https://chromiumcodereview.appspot.com/23022006, which was reverted due to perf regression. The root cause was WebContentsImpl::GetWebkitPrefs using GetLastCommittedEntry, which caused compositing to be disabled and higher memory usage on Mac. I've removed that part of the change and will investigate separately how to best fix it.

BUG=273710

Review URL: https://codereview.chromium.org/25654005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227665 0039d316-1c4b-4281-b951-d872f2087c98