Plumb composition character bounds from C++ to Java
This is a follow up CL of crrev.com/671503005.
This CL makes a plumbing from the C++ layer to Java layer so that
composition character bounds can be passed to new Android 5.0 IME APIs
in subsequent CLs.
As discussed in the previous CL, that callback is called in Android 5.0
and later. Hence no performance impact is intended in KitKat and prior.
All-in-one CL is available here: http://crrev.com/643193003
TEST=Manually done on Nexus 7 Build/LPX13D