Fix a warning.
[runtime] Don't define ucontext_t for android if PLATFORM is not android.
Implement the ReadEntityBodyMode properties
Implement Request.Abort()
Merge pull request #1353 from martinjt/ReadEntityBodyMode

Implement the ReadEntityBodyMode properties
Merge pull request #1354 from martinjt/RequestAbort

Implement Request.Abort()
[sgen] Fix allowance calculation.

The formula for calculating allowance was not only overly complicated
but completely broken, which resulted in the allowance always being the
minimum (4 times the nursery size).  That resulted in too frequent garbage
collections when the heap grew larger.

Now we do something very similar to what Boehm does: Allow the heap
to grow by a third before the next major collection happens.