Finalize rename of LayerImpl scroll offset methods.

Finalize rename of LayerImpl scroll offset methods.

This deletes stubs and renames all uses in tests for the following
methods that were renamed in https://codereview.chromium.org/800613009:

- LayerImpl::TotalScrollOffset was renamed to CurrentScrollOffset.
  TotalScrollOffset is now a LayerTreeImpl-specific concept meaning the
  sum of the inner and outer viewport current scroll offsets.
- LayerImpl::scroll_offset() was renamed to BaseScrollOffset.  This is
  the latest scroll offset pushed from the main thread.
- LayerImpl::SetScrollOffset() was renamed to
  PushScrollOffsetFromMainThread.  There are two ways to change the
  scroll offset, either pushing from the main thread or directly doing
  SetCurrentScrollOffset, corresponding to main-thread or impl-thread
  scrolling.

BUG=

Committed: https://crrev.com/f998da84f5813c2874d0fcc2e71b44e8d4c04314
Cr-Commit-Position: refs/heads/master@{#314238}

[aelias_OOO_until_Jul13, 2015-02-02 22:54:58]

aelias@chromium.org changed reviewers:
+ enne@chromium.org

[aelias_OOO_until_Jul13, 2015-02-02 22:55:18]

PTAL.

[enne (OOO), 2015-02-02 22:59:59]

lgtm

[aelias_OOO_until_Jul13, 2015-02-02 23:32:02]

The CQ bit was checked by aelias@chromium.org

[commit-bot: I haz the power, 2015-02-02 23:32:35]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/878413005/20001

[commit-bot: I haz the power, 2015-02-03 01:41:21]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-02-03 01:42:22]

Patchset 2 (id:??) landed as
https://crrev.com/f998da84f5813c2874d0fcc2e71b44e8d4c04314
Cr-Commit-Position: refs/heads/master@{#314238}