Change the cap on the amount of padding for each vtable to 32-byte (previously it was 128-byte)

We tested different cap values with a recent commit of Chromium. Our results show that the 32-byte cap yields the smallest binary and all the caps yield similar performance.
Based on the results, we propose to change the cap value to 32-byte.

Patch by Zhaomo Yang!

Differential Revision: https://reviews.llvm.org/D49405

llvm-svn: 337622