Trim IPC ChannelReader's buffer after processing large messages.
Most IPC messages are small, but there are few ones that can be quite large.
With this change ChannelReader trims its input buffer after processing large
messages.
BUG=
529940
Committed:
https://crrev.com/06a2e65a4f3610ec17dbc5988c0b16a95825240a
Cr-Commit-Position: refs/heads/master@{#357708}