Rename InstructionsReader to ImageReader for symmetry with ImageWriter.

runtime/vm/clustered_snapshot.cc

Index: runtime/vm/clustered_snapshot.cc
diff --git a/runtime/vm/clustered_snapshot.cc b/runtime/vm/clustered_snapshot.cc
index 11d9dd050935b76e8347b12e12a45f2de8e12989..6b7dbec93cc851e04c83483e72a22c96d01c8e7b 100644
--- a/runtime/vm/clustered_snapshot.cc
+++ b/runtime/vm/clustered_snapshot.cc
@@ -4952,7 +4952,7 @@ Deserializer::Deserializer(Thread* thread,
       zone_(thread->zone()),
       kind_(kind),
       stream_(buffer, size),
-      instructions_reader_(NULL),
+      image_reader_(NULL),
       refs_(NULL),
       next_ref_index_(1),
       clusters_(NULL) {
@@ -4960,8 +4960,7 @@ Deserializer::Deserializer(Thread* thread,
     ASSERT(instructions_buffer != NULL);
   }
   if (instructions_buffer != NULL) {
-    instructions_reader_ =
-        new (zone_) InstructionsReader(instructions_buffer, data_buffer);
+    image_reader_ = new (zone_) ImageReader(instructions_buffer, data_buffer);
   }
 }