Clarify FrameLoader's load starting functions

Rename loadItem() to loadHistoryItem()

Change reload() to take an enum instead of a boolean to indicate whether it is
an end to end reload.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/19765007

git-svn-id: svn://svn.chromium.org/blink/trunk@154661 bbb929c8-8fbe-4397-9dbb-9b2b20218538