Add a basic InkWell implementation

Add a basic InkWell implementation

This CL replaces the (non-working) components2 InkWell with some code based on
the ink_well example. There are at least two issues with the implementation:

1) The ink splash always starts at the center of the well because we don't have
   a facility for converting from global to local coordinates, which means we
   can't tell where the tap occurred in the local coordinates we need to use
   for painting.

2) When used inside a MenuItem, the in splash disappears shortly after
   starting, presumably because the button starts highlighting, which causes a
   component rebuild and somehow we lose the RenderInkWell instance.

I plan to address these issues in subsequent CLs.

R=ianh@google.com

Committed: https://chromium.googlesource.com/external/mojo/+/c81f8ce3200f69ed9b43745d78dc2560d9e875d3

[Hixie, 2015-06-10 15:56:35]

lgtm

[Hixie, 2015-06-10 15:56:47]

but put your constructors first, then your fields

[abarth-chromium, 2015-06-10 16:04:20]

Committed patchset #2 (id:20001) manually as
c81f8ce3200f69ed9b43745d78dc2560d9e875d3 (presubmit successful).