GitLab

Fixes a bug where ALL_OR_NONE writes to data pipes were checking the
total capacity of the pipe, rather than the available capacity. The bug
allowed ALL_OR_NONE writes to succeed (MOJO_RESULT_OK), but in actuality
only a portion of the data was written.

Updated unit test with a case that fails w/o the fix, but passes with
the fix.

Review-Url: https://codereview.chromium.org/2341633005
Cr-Commit-Position: refs/heads/master@{#418929}