Implement a MOJO_READ_DATA_FLAG_PEEK flag for data pipes.

mojo/edk/system/local_data_pipe.cc

Index: mojo/edk/system/local_data_pipe.cc
diff --git a/mojo/edk/system/local_data_pipe.cc b/mojo/edk/system/local_data_pipe.cc
index 8ee696cc8a1ac6f1fb5dc5e83350ba4211c2d151..fdfaf28cb2ab8f4809358c784077d8ca7a94775e 100644
--- a/mojo/edk/system/local_data_pipe.cc
+++ b/mojo/edk/system/local_data_pipe.cc
@@ -177,7 +177,8 @@ MojoResult LocalDataPipe::ConsumerReadDataImplNoLock(
     UserPointer<void> elements,
     UserPointer<uint32_t> num_bytes,
     uint32_t max_num_bytes_to_read,
-    uint32_t min_num_bytes_to_read) {
+    uint32_t min_num_bytes_to_read,
+    bool peek) {
   DCHECK_EQ(max_num_bytes_to_read % element_num_bytes(), 0u);
   DCHECK_EQ(min_num_bytes_to_read % element_num_bytes(), 0u);
   DCHECK_GT(max_num_bytes_to_read, 0u);
@@ -207,7 +208,8 @@ MojoResult LocalDataPipe::ConsumerReadDataImplNoLock(
         .PutArray(buffer_.get(), num_bytes_to_read - num_bytes_to_read_first);
   }
 
-  MarkDataAsConsumedNoLock(num_bytes_to_read);
+  if (!peek)
+    MarkDataAsConsumedNoLock(num_bytes_to_read);
   num_bytes.Put(static_cast<uint32_t>(num_bytes_to_read));
   return MOJO_RESULT_OK;
 }