Teach Widget some new tricks to enable build-during-layout.

Teach Widget some new tricks to enable build-during-layout.

There are basically three tricks here:
- Making retainStatefulNodeIfPossible non-private, so that
  RenderObjectWrappers in other files can use it.
- RenderObjectWrappers have a utility function that subclasses
  can call (retainStatefulRenderObjectWrapper()) to enable them
  to be retained, when they override retainStatefulNodeIfPossible.
- enterLayoutCallbackBuilder()/exitLayoutCallbackBuilder() API to set
  the right flags so that you can build Widget nodes outside a
  regular build cycle

Also:
- Widget.toStringName(), for debugging when you don't want the whole
  subtree printed.

R=abarth@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/8bc81ace0c3b6d145e976424293a7bf0af1ea82c

[abarth-chromium, 2015-07-09 00:41:24]

abarth@chromium.org changed reviewers:
+ abarth@chromium.org

[abarth-chromium, 2015-07-09 00:41:25]

lgtm

[Hixie, 2015-07-09 00:54:36]

Committed patchset #2 (id:20001) manually as
8bc81ace0c3b6d145e976424293a7bf0af1ea82c (presubmit successful).