Remove bogus use of the term "user style" in Blink and Blink API

The WebView::addUserStyleSheet() method in fact simply injects author styles
into the page. I've deprecated that method and replaced it with an injectStyleSheet()
method. Similar renaming has happened for the other levels of that codepath inside
Blink (in PageGroup, StyleEngine, TestRunner).

Also killed dead code supporting Page-level UserStyle injection, which isn't
possible in Blink/Chromium.

This patch makes no changes in behavior.

R=abarth

Review URL: https://codereview.chromium.org/25505007

git-svn-id: svn://svn.chromium.org/blink/trunk@158680 bbb929c8-8fbe-4397-9dbb-9b2b20218538