Change kMaxDescriptorsPerMessage back to 7

This is a partial revert of crrev.com/320831 which increased
Chrome's memory usage by 106KB per IPC::Channel.

BUG=484154
TEST=git cl try

Review URL: https://codereview.chromium.org/1127913002

Cr-Commit-Position: refs/heads/master@{#328587}
4 files changed