Make FixedHeightScrollable understand heights

Make FixedHeightScrollable understand heights

This CL makes FixedHeightScrollable able to measure heights. In the DOM-based
version, FixedHeightScrollable forced a synchronous layout in a microtask in
order to measure the size of itself and its children. In the DOM-less version,
we now require the subclass to tell us the item height and we measure our own
height using a layout callback.

R=ianh@google.com

Committed: https://chromium.googlesource.com/external/mojo/+/c35710c6b7a6452eb0551fe5f3c4015e0b26e2a2

[Hixie, 2015-06-04 20:35:50]

lgtm

https://codereview.chromium.org/1161063005/diff/1/sky/sdk/lib/framework/fn2.dart
File sky/sdk/lib/framework/fn2.dart (right):

https://codereview.chromium.org/1161063005/diff/1/sky/sdk/lib/framework/fn2.d...
sky/sdk/lib/framework/fn2.dart:437: }
Should probably drop the callback when the child is removed, too, in case the
RenderObject is actually used for something else later and ends up calling us
unexpectedly.

[abarth-chromium, 2015-06-04 20:41:25]

https://codereview.chromium.org/1161063005/diff/1/sky/sdk/lib/framework/fn2.dart
File sky/sdk/lib/framework/fn2.dart (right):

https://codereview.chromium.org/1161063005/diff/1/sky/sdk/lib/framework/fn2.d...
sky/sdk/lib/framework/fn2.dart:437: }
Done.

[abarth-chromium, 2015-06-04 20:42:20]

Committed patchset #1 (id:1) manually as
c35710c6b7a6452eb0551fe5f3c4015e0b26e2a2 (presubmit successful).