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If you go to a app that uses message pipe (e.g. the inner frame of
media_app) and run:
window.parent.postMessage('something', '*')
You get an error: "Uncaught TypeError: Cannot read property 'substr' of
undefined"

This happens because message pipe intercepts all postMessages and tries
to handle them.

This CL makes message pipe ignore messages where event.data.type isn't a
string.

This change is to help with migrating help app to use message pipe -- to
prevent the existing postMessages from causing errors.
http://crrev.com/c/2405044

Bug: b/166043485
Change-Id: Ia66259bd4de542d628228282503c1d00a51d3e10
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2413650
Commit-Queue: Zufeng Wang <zufeng@google.com>
Reviewed-by: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808781}