Add a comment to dex2oat's parent pid logging

Adds a comment explaining why we are logging parent pid in dex2oat.

Test: mm -j
Change-Id: I6ed56bb9315a264e8f3d98777e616c4c9e544506
diff --git a/dex2oat/dex2oat.cc b/dex2oat/dex2oat.cc
index 1296bcf..0fa27ea 100644
--- a/dex2oat/dex2oat.cc
+++ b/dex2oat/dex2oat.cc
@@ -2782,6 +2782,8 @@
     return EXIT_FAILURE;
   }
 
+  // Helps debugging on device. Can be used to determine which dalvikvm instance invoked a dex2oat
+  // instance. Used by tools/bisection_search/bisection_search.py.
   VLOG(compiler) << "Running dex2oat (parent PID = " << getppid() << ")";
 
   bool result;