Make Python not support "all or none" for two-phase data pipe read/write.

Make Python not support "all or none" for two-phase data pipe read/write.

Note: The data pipe tests are somewhat dependent on implementation at a
lower level. E.g., there's no guarantee that after a write, all the data
written becomes available "atomically". There's also no guarantee on the
size of the read/write buffers for two-phase reads/writes -- even if a
two-phase write of a given size was done, there's no guarantee that a
corresponding two-phase read will have a buffer of the same size
available. However, I've left the tests alone for now.

BUG=#366
R=vardhan@google.com

Committed: https://chromium.googlesource.com/external/mojo/+/7e232ee45eefa05a19e137850377e6151a260e88

[vardhan, 2015-09-25 21:41:44]

lgtm

[vardhan, 2015-09-25 21:41:45]

lgtm

[viettrungluu, 2015-09-25 22:04:02]

Committed patchset #1 (id:1) manually as
7e232ee45eefa05a19e137850377e6151a260e88 (presubmit successful).