This CL implements a basic version of screen rotation where we just relayout at
the new size. In the future, we'll want a fancy version where we transition
between the two layouts.
R=ianh@google.com
BUG=
Review URL: https://codereview.chromium.org/1177563002.
- makes the event logic not involve a boolean return value (since we ignored it anyway)
- splits the event handling logic into two steps, hit testing and event dispatch
- introduces an App class on the Dart side to factor out the interaction with the C++ side
- ports sector-layout and simple_render_tree to the new App infrastructure
- port simple_render_tree to the new event handling logic
- implement hit testing for the sector-layout demo
R=eseidel@chromium.org
Review URL: https://codereview.chromium.org/1143343004
