Do not iterate past the end on processing leaf class.

When checking that the leftmost class of a GC-derived class has the
expected shape, take care not step past the end of the leftmost class'
own base classes, i.e., when processing A in

  class A { };
  class B : public A, ...some GC base... { };

R=haraken
BUG=446580

Review URL: https://codereview.chromium.org/840623002

Cr-Commit-Position: refs/heads/master@{#310253}
3 files changed