Revert "Revert 157959 "Introduce a new reference graph to SVG*TearOffs.""

Reland the patches. All known SVGProperty memory bugs were resolved.

BUG=270000
BUG=294464

TBR=pdr@chromium.org,haraken@chromium.org,inferno@chromium.org

Review URL: https://codereview.chromium.org/25461003

git-svn-id: svn://svn.chromium.org/blink/trunk@158679 bbb929c8-8fbe-4397-9dbb-9b2b20218538
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