Fix google-explicit-constructor warnings in art.

* Add explicit keyword to conversion constructors,
  or NOLINT for implicit converters.
Bug: 28341362
Test: build with WITH_TIDY=1

Change-Id: I1e1ee2661812944904fedadeff97b620506db47d
diff --git a/runtime/oat.h b/runtime/oat.h
index 35d0c92..12a8298 100644
--- a/runtime/oat.h
+++ b/runtime/oat.h
@@ -171,7 +171,7 @@
 
 class PACKED(4) OatMethodOffsets {
  public:
-  OatMethodOffsets(uint32_t code_offset = 0);
+  explicit OatMethodOffsets(uint32_t code_offset = 0);
 
   ~OatMethodOffsets();