Remove RenderLayer::scrollTo* functions

Added RenderBox::scrollToOffset for the accessibility code.

BUG=260899

Review URL: https://codereview.chromium.org/26528003

git-svn-id: svn://svn.chromium.org/blink/trunk@159840 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Clean up Document/ExecutionContext lifetime management in html/track/*

Nearly all the classes under html/track previously took ExecutionContext
pointers in their constructors, and stored them as members without any
explanation of how the lifetimes of those pointers were supposed to work.

There were generally two uses for those pointers:
  - To have something to return for EventTarget::executionContext()
  - To treat as a Document for the construction of DOM nodes

In this patch, the main thing you'll notice is a lack of toDocument()
calls. Any class that needs a Document now gets one, either in its
constructor or in the methods that require that Document to do work.

Some of those Documents are later returned from executionContext(),
while in other places I found a different place to get the executionContext
(e.g., from an Element that the class was already holding a reference to).

I tried to avoid holding a member pointer back to the Document where possible,
but in a few cases I had to keep it for now. A future patch will aim to
get rid of more Document pointers, as their lifetimes look questionable
(though I know of no existing bugs with those pointers).

Also done in this patch, but only marginally related to the above:
  - Simplified TrackBase to only provide a type tag and no other functionality
  - Changed TrackBase::Type to include only valid enum values

R=acolwell@chromium.org, vcarbune@chromium.org

Review URL: https://codereview.chromium.org/26849005

git-svn-id: svn://svn.chromium.org/blink/trunk@159841 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove BackForwardController.

Have HistoryController do all tracking of the current HistoryItem. In the
rest of its uses, have callers talk directly to BackForwardClient, which
now lives directly on Page.

BUG=none

Review URL: https://codereview.chromium.org/27024004

git-svn-id: svn://svn.chromium.org/blink/trunk@159842 bbb929c8-8fbe-4397-9dbb-9b2b20218538