[tools/logreader] Print overflow drops to stderr

Don't trash stdout with "dropped: overflow" messages (or other errors)
in the log reader, which then cause generated json files to fail to be
read by other tools.

Change-Id: Ie27639dbbee6fc9e8da0bc6901667c3a2835fbef
Reviewed-on: https://chromium-review.googlesource.com/456499Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43901}

tools/logreader.js

@@ -158,7 +158,7 @@ LogReader.prototype.processStack = function(pc, func, stack) {
     } else if (firstChar != 'o') {
       fullStack.push(parseInt(frame, 16));
     } else {
-      print("dropping: " + frame);
+      this.printError("dropping: " + frame);
     }
   }
   return fullStack;