Clean up MappingTable::FirstDexToPcPtr().

Use DecodeSignedLeb128() to skip over dex PCs. This is just
to be consistent, the number of bytes skipped is the same
as with DecodeUnsignedLeb128() and the result is ignored.

Change-Id: Ib5198b0367030957a6e2c650200121c2d828c0a6
diff --git a/runtime/mapping_table.h b/runtime/mapping_table.h
index a82bc1c..79e6e94 100644
--- a/runtime/mapping_table.h
+++ b/runtime/mapping_table.h
@@ -56,8 +56,8 @@
       // We must have dex to pc entries or else the loop will go beyond the end of the table.
       DCHECK_GT(total_size, pc_to_dex_size);
       for (uint32_t i = 0; i < pc_to_dex_size; ++i) {
-        DecodeUnsignedLeb128(&table);  // Move ptr past native PC.
-        DecodeUnsignedLeb128(&table);  // Move ptr past dex PC.
+        DecodeUnsignedLeb128(&table);  // Move ptr past native PC delta.
+        DecodeSignedLeb128(&table);  // Move ptr past dex PC delta.
       }
     }
     return table;