Make popup menus line up to their baseline per the Material spec.

Make popup menus line up to their baseline per the Material spec.

This entails:
 - Making the baseline logic cache results.
 - Making the baseline logic track who used its information.
 - Making the baseline logic mark all ancestors up to whoever used
   its information wheneven its node gets markNeedsLayout.
 - Making RenderShrinkWrapWidth make its child respect the step width
   and step height, rather than just sizing the child then snapping.
   This is required to make the ink splashes render right on menus
   that are snapped.
 - Adding debugDescribeSettings() to RenderShrinkWrapWidth.
 - Introducing a RenderBaseline class that offsets its child to a
   certain baseline.
 - Factoring out some common code from RenderBaseline and
   RenderPositionedBox to RenderShiftedBox.
 - Redoing all the menu layout logic.

BUG=
R=abarth@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/b5dcf78f609212eced8d6657182e9cc5e98619ce

[abarth-chromium, 2015-07-01 17:29:53]

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

[abarth-chromium, 2015-07-01 17:29:53]

https://codereview.chromium.org/1219113003/diff/80001/sky/sdk/lib/rendering/b...
File sky/sdk/lib/rendering/box.dart (right):

https://codereview.chromium.org/1219113003/diff/80001/sky/sdk/lib/rendering/b...
sky/sdk/lib/rendering/box.dart:334: _cachedBaselines[baseline] =
computeDistanceToActualBaseline(baseline, client);
_cachedBaselines.putIfAbsent(...)

[abarth-chromium, 2015-07-01 18:22:59]

lgtm

https://codereview.chromium.org/1219113003/diff/100001/sky/sdk/lib/rendering/...
File sky/sdk/lib/rendering/box.dart (right):

https://codereview.chromium.org/1219113003/diff/100001/sky/sdk/lib/rendering/...
sky/sdk/lib/rendering/box.dart:368: RenderBox node = this;
Is it possible to execute this line of code?

[Hixie, 2015-07-01 19:24:49]

Committed patchset #7 (id:120001) manually as
b5dcf78f609212eced8d6657182e9cc5e98619ce (presubmit successful).