Make IPC::Channel buffers stack based and secure against growth

Auxiliary IPC::Channel buffers have been permanently allocated
even though their use is very temporary. This moves those to
the stack instead to reflect their temporary nature and also
adds an assert to catch accidental out-of-control growth of
the buffer as happened recently.

BUG=484154
R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/1120343002

Cr-Commit-Position: refs/heads/master@{#331956}
2 files changed