Simplify indexed back/forward navigations

Before this patch:
* Something calls goBack, goForward, or goBackOrForward on BackForwardController.
* BackForwardClientImpl creates a fake HistoryItem with a url of the form chrome-back-forward://go/<offset> and returns it to BackFowardController
* BackForwardController passes that fake HistoryItem to HistoryController::goToItem through Page::goToItem.
* HistoryController::goToItem calls FrameLoaderClient::shouldGoToHistoryItem, which decodes the fake HistoryItem's url, gets the offset we want to navigate, and passes that to WebViewClient::navigationBackForwardSoon.

After this patch:
* Something calls goBackOrForward on BackForwardController.
* BackForwardController calls FrameLoaderClient::navigationBackForward() with the integer offset, which in turn calls WebViewClient::navigationBackForwardSoon

BUG=none

Review URL: https://chromiumcodereview.appspot.com/23122002

git-svn-id: svn://svn.chromium.org/blink/trunk@156104 bbb929c8-8fbe-4397-9dbb-9b2b20218538