Provide structures from <ucontext.h> for Android.
Provide structures from <ucontext.h> for Android.

Patch from Ben Murdoch <benm@chromium.org>.

Review URL: https://chromiumcodereview.appspot.com/10001005

git-svn-id: b158db1e4b4ab85d4c9e510fdef4b1e8c614b15b@11328 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh