Temporarily add UMA histogram for IPC message sizes.

Most messages are small, but there are few huge ones. Huge messages
inflate IPC buffers wasting memory. We want to shrink IPC buffers
after processing huge messages, so we need to find out best size to
shrink to.

BUG=529940

Review URL: https://codereview.chromium.org/1378953002

Cr-Commit-Position: refs/heads/master@{#354047}
2 files changed