Have ScratchBuffer::Allocate prefer allocating on the stack even if a previous

mojo/public/tests/buffer_unittest.cc

Index: mojo/public/tests/buffer_unittest.cc
diff --git a/mojo/public/tests/buffer_unittest.cc b/mojo/public/tests/buffer_unittest.cc
index 7699962a30697298e38d9f639090caa4fac2c56d..3043b135c88458252053b1a4b34aeaa515c0cbd6 100644
--- a/mojo/public/tests/buffer_unittest.cc
+++ b/mojo/public/tests/buffer_unittest.cc
@@ -32,10 +32,9 @@ TEST(ScratchBufferTest, Basic) {
   EXPECT_FALSE(large >= &buf && large < (&buf + sizeof(buf)));
 
   // But another small allocation should be back on the stack.
-  // TODO(mpcomplete): but it isn't. We can fix that easily enough.
   small = buf.Allocate(10);
   EXPECT_TRUE(IsZero(small, 10));
-  //EXPECT_TRUE(small >= &buf && small < (&buf + sizeof(buf)));
+  EXPECT_TRUE(small >= &buf && small < (&buf + sizeof(buf)));
 }
 
 // Tests that FixedBuffer allocates memory aligned to 8 byte boundaries.