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Enforce a maximum message limit for extensions of a 64MB string (from
the JSON-ified object passed to the API call). We enforce a 128 MB limit
on IPC messages, but even that should be excessively large for our needs.
Also add metrics to see what average messages look like - 64MB is *huge*
(and we have to serialize/deserialize the object multiple times, so
message size can have a pretty big performance impact), so we may be
able to reduce this furter.

Add a test for the new behavior.

Bug: 766713

Change-Id: Ic72b2c6c3ea3ca68a42fcd8061c9e006bc6f1795
Reviewed-on: https://chromium-review.googlesource.com/688674Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505176}