Rename DocumentThreadableLoaderClient::willSendRequest() to willFollowRedirect()

The method is for handling redirect responses. It's confusing that this
is given a name sounds like intercepting all requests.

R=mkwst
BUG=none

Review URL: https://codereview.chromium.org/679023002

git-svn-id: svn://svn.chromium.org/blink/trunk@184429 bbb929c8-8fbe-4397-9dbb-9b2b20218538