Mojo: Reduce default message allocation size

4 kB is a bit large. Microbenchmarks don't indicate a performance
hit when lowering this, and they may even improve on lower-end
devices.

This decreases the default size to 128 bytes. We can monitor perf
dashboards for the effect.

Also fixes a bug in test code which assumed an allocation wouldn't
resize a message, since that actually happens now.

Bug: None
Change-Id: Ibc7d17fe6f7cc98db7b290862cd4aea0f9be4fe0
Reviewed-on: https://chromium-review.googlesource.com/992553Reviewed-by: Jay Civelli <jcivelli@chromium.org>
Commit-Queue: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548079}

mojo/edk/system/user_message_impl.cc

@@ -33,7 +33,7 @@ namespace {
 // The minimum amount of memory to allocate for a new serialized message buffer.
 // This should be sufficiently large such that most seiralized messages do not
 // incur any reallocations as they're expanded to full size.
-const uint32_t kMinimumPayloadBufferSize = 4096;
+const uint32_t kMinimumPayloadBufferSize = 128;
 
 // Indicates whether handle serialization failure should be emulated in testing.
 bool g_always_fail_handle_serialization = false;

mojo/public/cpp/bindings/tests/struct_unittest.cc

@@ -82,7 +82,8 @@ U SerializeAndDeserialize(T input) {
   void* subsequent_area = message.payload_buffer()->AllocateAndGet(32);
   memset(subsequent_area, 0xAA, 32);
 
-  OutputDataType output_data = reinterpret_cast<OutputDataType>(data);
+  OutputDataType output_data =
+      reinterpret_cast<OutputDataType>(message.mutable_payload());
 
   U output;
   mojo::internal::Deserialize<OutputMojomType>(output_data, &output, &context);