Mojo: Fix data pipe ALL_OR_NONE write capacity check.

mojo/edk/system/data_pipe_unittest.cc

Index: mojo/edk/system/data_pipe_unittest.cc
diff --git a/mojo/edk/system/data_pipe_unittest.cc b/mojo/edk/system/data_pipe_unittest.cc
index 6f8362e69c4a9607626dc7c942a549ebebb8e381..32e661e7f5291ba197276ffd2ddada06fc09c856 100644
--- a/mojo/edk/system/data_pipe_unittest.cc
+++ b/mojo/edk/system/data_pipe_unittest.cc
@@ -799,7 +799,7 @@ TEST_F(DataPipeTest, AllOrNone) {
   ASSERT_EQ(MOJO_RESULT_OK, Create(&options));
   MojoHandleSignalsState hss;
 
-  // Try writing way too much.
+  // Try writing more than the total capacity of the pipe.
   uint32_t num_bytes = 20u * sizeof(int32_t);
   int32_t buffer[100];
   Seq(0, arraysize(buffer), buffer);
@@ -834,12 +834,11 @@ TEST_F(DataPipeTest, AllOrNone) {
   ASSERT_EQ(MOJO_RESULT_OK, QueryData(&num_bytes));
   ASSERT_EQ(5u * sizeof(int32_t), num_bytes);
 
-  /* TODO(jam): enable if we end up observing max capacity

[miu, 2016/09/14 22:40:23]

Not sure what the TODO comment was talking about here, but I resurrected the
commented-out test code. Please advise if there's anything extra you want me to
do here.

-  // Too much.
+  // Try writing more than the available capacity of the pipe, but less than the
+  // total capacity.
   num_bytes = 6u * sizeof(int32_t);
   Seq(200, arraysize(buffer), buffer);
   ASSERT_EQ(MOJO_RESULT_OUT_OF_RANGE, WriteData(buffer, &num_bytes, true));
-  */
 
   // Try reading too much.
   num_bytes = 11u * sizeof(int32_t);